LOW EVEN-ORDER HARMONIC DISTORTION AMPLIFIER AND METHODTHEREFOR

Abstract: An amplifying system and method produces a substan­ tial reduction in the D. C. and even-order harmonics in an output signal (x₀) by employing a first inverting amplifying stage (22) cascaded with an attenuating stage (26) which is cascaded with a second inverting amplifying stage (30). The electrical characteristics of each of the two inverting amplify­ ing stages (22,30) are substantially the same. The gain, in dB, exhibited by each of the amplifying stages (22,30) is substantially equal to the attenuation loss, in dB, produced by the attenuator stage (26). Conventional components and fabrication techniques allow substantial attenuation of the D. C. and even-order harmonic components produced by the non-linearities in the active devices of the amplifiers as compared to a single amplifier case producing the same gain A.

Description: The present invention relates generally to amplifying apparatus and methods, and more particularly, to amplifying apparatus and methods which reduce significantly even-order harmonic distortion. Active devices used in amplifiers typically exhibit nonlinearities. As is well-known, such nonlinearities produce unwanted components in the signal at the output of the amplifier. This is particularly the case with amplifiers using semiconductor active devices. Such amplifiers typically produce unwanted D.C. and even and odd order harmonics in the output signal. Representative of conventional amplifying systems is that shown in United States Patent 3,399,356 to Davies. The Davies circuit shows a system employing at least two cascaded tuned emitter stages with an attenuator as a coupling circuit between the stages. The Davies circuit teaches the use of tuned emitter amplifier stages; broad band amplifiers are not taught. The stages are inverting. Further, the relative gains of the amplifiers and the attenuator are not discussed. The Davies system does not address reduction of unwanted harmonic components. Another patent that is representative of conventional amplifiers is United States Patent 3,997,852 to Schornack et al . The Schornack patent shows an r.f. wide band amplifier. An input signal is provided to a transformer, whose output is provided to a field effect transistor attenuator. The output of the field effect transistor attenuator is applied to the input of a field effect transistor amplifier, whose output is provided to a transformer. The output of the amplifier system is provided at the output of this second transformer. The Schornack system only employs one amplifier. The inventor is also aware of the following additional United States patents which are representative of conventional amplifier technology: 1,646,438 to Affel; 2,440,465 to Ferguson; 2,617,039 to Young; 2,839,619 to Johnstone; 2,840,646 to Bell; 3,076,133 to Holcomb; 3,246,252 to Beyer; 3,633,120 to Battjes; 3,729,683 to Hemme; 3,863,173 to Scheid et al. ; 4,074,204 to Broburg et al. ; 4,155,047 to Rubens et al. ; 4,301,445 to Robinson; 4,367,442 to Tanaka et al. ; and 4,379,995 to Yamada et al. It is known from US Patent No 4237426 (Sueyoshi) to reduce distortion in a transistor amplifier without using feedback or large numbers of stages, by providing a front inverting amplifier for inverting an input signal, and a rear inverting amplifier for inverting that inverted signal to provide an amplified output. The transistors in each amplifier are of equal characteristics; the resistors in the front amplifier are of equal value or have an integral ratio (n) of values, and the number of transistors in the rear amplifier is n, Darlington-connected together. Specific examples of this general arrangement will reduce even-order harmonic distortion somewhat for a limited range of input signal voltage levels. The purpose of the present invention is to enable even-order distortion to be reduced using amplifiers of any configuration and of any arbitrary gain, and this is achieved in the apparatus defined in Claim 1 and in the amplifying method defined in Claim 8. It should be noted that gains and attenuations are expressed as multiplying factors throughout this discussion, and are expressed in dB only if specifically noted. The attenuator means typically exhibits a broad band frequency response characteristic. The output from the second amplifier means has a gain substantially equal to a gain of A compared to the input signal provided to the first amplifier means, but its D.C. component and its even-order frequency components are substantially attenuated. In this way, two inverting amplifiers can be cascaded with an attenuator stage disposed between them to provide superior attenuation of the D.C. and even-order components as compared to a single amplifier stage which amplifies an input signal by the same gain amount -A. More specifically, the first and second inverting amplifier means can utilize semiconductor active devices such as bipolar transistors. Further, any suitable type of amplifier configuration can be used as long as it provides the desired substantially identical electrical performance. Such amplifier stages can be fabricated on a single substrate in the form of a hybrid circuit or the active devices of such amplifier stages can utilize the same substrate. Such an approach results in the amplifiers exhibiting substantially the same electrical characteristics if all external components are properly matched. The attenuator means that is interposed between the first inverting amplifier means and the second inverting amplifier means can take any form that will produce the desired attenuation of 1/A. Representative of such suitable attenuators are resistive elements and power dividers. When a power divider is employed, the excess power produced by such device can be utilized by a utilization device to effect desired functions or controls of other portions of an electronic system. For example, the utilization device can be a detector diode which produces an output signal used to control the level of the output of an instrument which utilizes the output from the second amplifier stage. Substantial reduction in the D.C. and even-order harmonics can be achieved in practice using conventional parts and fabrication techniques. While the use of such conventional parts and fabrication techniques does not result in the reduction in the D.C. and even-order harmonic components that is predicted theoretically, substantial reduction has been achieved in practice without the requirement of special testing or matching of components. In this way, the present invention has particular applicability for use in the mass production of amplifiers used in, for example, test equipment and the like where criticalness in the operation of an electronic circuit must be minimized in order to achieve reliable performance in the field. The present invention can operate in any desired frequency range, such as audio frequency or radio frequency. Theoretically, the present invention can be utilized wherever an active device is used in each of the two amplifying stages. Various objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description of embodiments of the present invention given by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the mathematical representation of the single amplifier case; Figure 2 is a block diagram of the mathematical representation of the first amplifier followed by an attenuator case; Figure 3 is a block diagram of the mathematical representation of the noninverting amplifier followed by an attenuator follower by a noninverting amplifier case; Figure 4 is a block diagram of the mathematical representation of the present invention comprising an inverting amplifier following by an attenuator followed by an inverting amplifier. Figure 5 is a schematic diagram of a representative inverting semiconductor amplifier which can be utilized for each of the two inverting amplifying stages of the present invention; Figure 6 is a block diagram of a test set-up utilized to produce experimental data showing the operation of the present invention; Figure 7 is a graph plotting the signal present at point A of the block diagram of Figure 6, where the horizontal axis represents frequency in MHz and the vertical axis represents amplitude in dB; Figure 8 is a graph plotting the signal present at test point B of Figure 6, where the vertical and horizontal axes are the same as in Figure 7; Figure 9 is a graph of the plot of the signal present at test point C of Figure 6, where the vertical and horizontal axes are the same as those of Figure 7; Figure l0 is a plot of the output present at test point D of Figure 6, where the vertical and horizontal axes are the same as those axes of Figure 7; and Figure ll is a block diagram of the present invention showing the addition of a utilization device coupled to the output of the signal splitter. The full dimensions of the present invention can be appreciated by reviewing the theory of operation of related circuit topologies. This review is presented below using mathematical models of such related circuit topologies. Before reviewing the related circuit topologies, it may be useful to understand the present invention in broad terms. In summary, the present invention comprises a first amplifier exhibiting substantially a gain of-A. The amplifier is of the inverting type. An input signal is amplified by the first amplifier and provided at its output. The signal at the output of the first amplifier is supplied to an attenuator. The attenuator exhibits an attenuation substantially of l/A. The attenuated output of the attenuator--which has a level substantially equal to the input level of the first amplifier--is applied to the input of a second amplifier. The second amplifier inverts the attenuated output and amplifies it by a gain substantially equal to -A. Like the first amplifier, the second amplifier is of the inverting type. The output signal from the second amplifier has a level substantially equal to the level of the input signal applied to the first amplifier increased by a gain A. While one would expect at best no increase, and at worst an increase, in the distortion components present in the output of the first amplifier, the inventor has found unexpectedly that this is not the case. Instead, substantial attenuation of the D.C. and even-order harmonics is present in the output of the second amplifier as compared to the levels of such components in the output of a single amplifier of gain -A. In this way, the present invention using standard active devices in standard amplifier circuits can produce an amplified output signal having a substantial reduction in the D.C. and even-order harmonics. Turning now to related circuit topologies, reference is now made to Figure l. Figure l shows a block diagram representative of a mathematical model of a single amplifier. For purposes of discussion, this shall be called the single amplifier case. Several assumptions have been made concerning the single amplifier case. First, it is assumed that the amplifier exhibits amplitude distortion, which results in a non-linear transfer characteristic. Second, it is assumed that the amplifier is memoryless. This means that the output of the amplifier is an instantaneous function of the input. In other words, frequency response effects are not considered in the mathematical model. Third, it is assumed that the nonlinearity exhibited by the amplifier is weak. As an aside, it is well-known that the operation of an amplifier can be modeled mathematically by a power series of the input signal. A suitable reference source which sets forth such mathematical modeling is Solid State Microwave Amplifier Design by Tri T. Ha, published in 1981 by John Wiley & Sons. Equation I shown below is the transfer function of the single amplifier case where the nonlinearity is weak, as shown by equation II, for the range of the input signal x i of interest: where: the linear term = α₁x i the second harmonic term = α₂x 2 / i the third harmonic term = α₃x 3 / i Note that the transfer function shown in equation I only includes the linear term, the second harmonic term and the third harmonic term. This is due to the fact that the nonlinearity is weak, as shown in equation II. In other words, the fourth and higher order terms can be ignored for purposes of this analysis since they are of such low value. When a sine wave input signal of equation III below is applied to the input of the single amplifier case, it produces the output signal shown in equation IV, which has been rewritten in equation V for purposes of simplification: x i = V o coswt (III) x o = β o +β₁coswt+β₂cosw2t+β₃cos3wt (V) where β coefficients are in terms of α coefficients and the input signal amplitude V o , as follows: Note that the output signal in the single amplifier case contains the fundamental frequency as well as D.C., second and third harmonic components. Also note that the D.C., second and third harmonic components can be of significant value relative to the fundamental frequency components if the α₂ and α₂ values are significant. Next, the single amplifier followed by an attenuator case is examined. Referring now to Figure 2, a block diagram of the mathematical model used to represent an attenuator connected to the output of the single amplifier is shown. The amplifier is the same as the amplifier of the single amplifier case. Thus, the equations used for the single amplifier case apply with respect to the amplifier of the single amplifier followed by an attenuator case. The attenuator exhibits the transfer function shown below in equation VI: g(x) = l A x (VI) where: l/A = attenutation factor Equation VII below shows the transfer function of the single amplifier followed by an attenuator case: The output x o provided by the attenuator is shown below by equation VIII where the input signal is the sine wave of equation III and the β's are those of equation V: x o = l A [β o +β₁coswt+β₂cos2wt+β₃cos3wt] (VIII) Equation VIII shows that the ratio of the D.C., second and third harmonics to the fundamental is the same as that present in the single amplifier case, except that all are equally attenuated by an amount equal to the value of attenuation l/A. Conceptually, this makes sense since the attenuation is assumed to be frequency independent. In other words, an equal amount of attenuation of the D.C., fundamental, second and third harmonics occurs. Referring now to Figure 3, the case of an amplifier followed by an attenuator followed by an identical amplifier is shown in block form, which represents the mathematical model of such case. It is assumed that the two amplifiers are substantially the same and are like the amplifier in the single amplifier case and the single amplifier followed by an attenuator case discussed above. The input of the second amplifier, which is the output of the attenuator, is shown below by equation IX. Note that equation IX is the same as equation VII discussed above for the amplifier followed by an attenuator case: Equation X shown below is the transfer function for the output of the second amplifier. It is obtained by substituting equation IX into equation I discussed above: An approximation is now made to simplify the analysis. Specifically, as shown in equation II above, the nonlinear terms are substantially less than the linear term for the range of interest of x i . Since this is the case, an approximation expression for x o of equation X can be rewritten and retains the dominant terms. This equation is shown below as equation XI: Equation XI is rewritten as a power series in equation XII as follows: Now, applying a sine wave term in the form of equation III above into equation XII produces the approximation of the output signal shown below in equation XIII: Equation XIII can be rewritten as equation XIV shown below, which is in the form of a sum of harmonic components as follows: x o = γ o +γ₁coswt+γ₂cos2wt+γ₃cos3wt (XIV) γ coefficients are expressed in terms of α coefficients of equation I and the input signal amplitude V o as follows: Equation XIV shows that the output signal from the second amplifier includes a D.C. component, a second harmonic component, and a third harmonic component in addition to the desired fundamental signal. Equation XIV also shows that these undesired components may be of significant value if the coefficients that multiply them have a significant value, which often is the case. Assume now, for purposes of analysis, that the noninverting amplifiers have equal gain and that the gain of each, in dB, is equal to the attenuator's attenuation, in dB. In other words, assume in this special case, that α₁ = A. Substituting A into equation XIV produces the following γ coefficients shown below as equation XV: Substituting the γ terms of equation XV into equation XIV produces equation XVI shown below. Equation XVI thus shows the output signal x o provided by the second amplifier in the amplifier followed by attenuator followed by an amplifier case where the amplification gain of each amplifier equals A and the attenuation factor produced by the attenuator equals l/A. Compare this with equation V (which is reproduced below for convenience purposes), which shows the output signal provided by the single amplifier case. x o = β o +β₁coswt+β₂cos2wt+β₃cos3wt (V) x o = 2β o +β₁coswt+2β₂cos2wt+2β₃cos3wt (XVI) It is thus seen that in this special case of the amplifier followed by an attenuator followed by an amplifier situation that the output signal has been degraded, in that its D.C. component, second harmonic component and third harmonic components are twice what they would be if only a single amplifier case had been used. This makes sense since the nonlinear components produced by each amplifier are doubled when two amplifiers are cascaded. Obviously, the attenuator eliminates the gain produced by one of the single amplifiers since it has a attenuation equal to l/A, which is the reciprocal of the gain of a single amplifier. The present invention comprises in its basic form an inverting amplifier followed by an attenuator followed by an inverting amplifier, which is shown in block diagram form in Figure 4. When the amplifiers are made substantially the same and the gain of each is equal to approximately -A and the attenuation exhibited by the attenuator is equal to l/A, the inventor has discovered that a substantial reduction in the D.C. and even-order harmonics occurs in the output signal. This is very surprising when you compare this result with that produced by the special case of the noninverting amplifier followed by an attenuator followed by an noninverting amplifier presented above in equation XVI. The mathematical analysis presented below shows why the results obtained with the present invention occur; following this technical explanation is a representative example of the present invention which shows actual test results that confirm this theoretical analysis. Equation XVII below shows the results of substituting into equation XIV the assumptions that α₁ equals -A: Substituting the values obtained for equation XVII into equation XIV (presented above) produces equation XVIII shown below, which represents the output x o of the second inverting amplifier as provided by the present invention. Equation XVII is compared with equation V (which is the equation for the single amplifier case; it is repeated below for purposes of comparison), and shows that the D.C. and second harmonic are cancelled out by the present invention and that the third harmonic is twice the level that the third harmonic is in the single amplifier case: x o = β o +β₁coswt+β₂cos2wt+β₃cos3wt (V) x o = -β₁coswt-2β₃cos3wt (XVIII) Note that the cancellation of the D.C. and the second harmonic component occurs only if the relationship presented below as equation XIX occurs: Thus, the present invention requires that α₁ = -A. To reiterate, this means that the amplifiers of the present invention must: (l) each have gain (in dB) equal to the attenuator's loss (in dB); (2) each be substantially identical; and (3) each be inverting. All three of these conditions must be met in order for the present invention to provide the low level D.C. and even-order harmonic distortion characteristic. Equation XX is the end result of a more general analysis, where the amplifier's transfer characteristic is assumed to be: f(x) = α₁x+α₂x²+α₃x³+α₄x⁴+... Note that as is the case with equation XVIII above, that while the even-order harmonics are suppressed, the present invention produces odd order harmonics that are twice the level of the single amplifier case: In summary, the present invention produces a substantial reduction in the D.C. and the even-order harmonics as compared to the output of a single amplifier producing the same amount of gain. In contrast, the present invention increases by 6 dB the level of the odd order harmonics that are present in its output signal. Thus, the present invention has particular application in situations where it is desired to substantially reduce the D.C. and even-order harmonics (particularly the second order harmonic) present in an output signal. It can be appreciated that the present invention does not require the use of precision inverting amplifiers or a precision attenuator to achieve the desired results, as is exhibited below in the representative example of the present invention which is now discussed. Figure 5 shows a schematic of a suitable amplifier that can be employed for each of the two inverting amplifier stages of the present invention. It should be understood clearly, however, that the amplifier shown in Figure 5 is for purposes of illustration only; any type of amplifier can be employed that meets the three criteria set forth above in order to produce the desired suppression of the the D.C. and even-order harmonics produced by the present invention. The amplifier shown in Figure 5 includes representative parts values; it produces a l2 dB gain from .l MHz to 250 MHz. Input and output impedances are nominally 50 ohms. The amplifier is a common-emitter type, with base-collector feedback and emitter degeneration. Figure 6 shows a test set-up in block diagram form which is used to provide the experimental verification presented below showing the even-order harmonic cancellation produced by the present invention. A signal source l0 is used to generate a test signal. A suitable form for signal source l0 is a Fluke Model 607lA, manufactured by the applicant. The output of signal source l0 is provided via a coaxial line l2 to the input of a low pass filter l4 of conventional design. Low pass filter l4 has a predetermined cutoff frequency. In the present situation, where the signal on line l2 has a frequency of 30 MHz, low pass filter l4 can be made to have a cutoff substantially equal to the frequency of the input signal on line 12 (i.e., 30 MHz). The output from the low pass filter stage l4 is provided via a coaxial line l6 to an attenuator l8. Any type of attenuator can be employed. Note that there is no requirement that attenuator l8 be used. It was employed in the test set-up in order to control the output of the signal on line l6. A suitable example for attenuator l8 is a -3 dB resistive attenuator such as a Narda Model 478l-3. The Narda Microwave Corporation is located in Plainview, New York. The output from attenuator l8 is provided via a coaxial line 20 to the first inverting amplifier 22. As stated above, amplifier 22 can be of any suitable type. In the test set-up, amplifier 22 was of the type shown in Figure 5 above, and exhibited a l2 dB gain over the bandwidth set forth above. Note that reference A represents the test point at which the first spectrum analyzer plot (Figure 7 discussed below) was obtained. The output of the first amplifier 22 is provided via a coaxial line 24 to the input of an attenuator 26. Attenuator 26 can be of any suitable type which provides the desired attenuation (in this case, l2 dB). A resistive attenuator can be employed, as can a power divider. All that is required is that the attenuation of a desired value is produced in the frequency band of interest. As shown with respect to Figure ll below, the excess power produced by a power divider (as compared to a resistive divider which dissipates the excess power through heat dissipation) can be employed to control any type of utilization device or circuit function that is desired. Reference B represents the test point at which the second spectrum analyzer plot (Figure 8) is taken. Similarly, reference C represents the test point at which the third spectrum analyzer plot was taken (Figure 9). A suitable example for attenuator 26 is a variable attenuator Model 907A-20ll made by Weinschel Engineering. Weinschel is located in Gaithersburg, Md. The attenuator is set for -l2 dB attenuation. The output of the attenuator 26 on a coaxial line 28 is applied to a input of a second inverting amplifier 30. Second inverting amplifier 30 is identical to first inverting amplifier 22. In the test set-up, amplifier 30 thus was of the form of the amplifier shown in Figure 5 above, and produced a l2 dB gain. The output of second amplifier 30 on a coaxial line 32 is applied to the input of a spectrum analyzer 34. Test point D is where the fourth spectrum analyzer plot (Figure l0 below) was taken. A suitable form for spectrum analyzer 34 is a Hewlett Packard Model 8568A. Hewlett Packard is located in Palo Alto, California. Referring now to Figures 7-l0, the test results of the test set-up shown in Figure 6 are shown, where the input signal to the first amplifier is 6.0 dBm at 30 MHz. Referring first to Figure 7, this plots the spectrum of the signal provided to the input of first inverting amplifier 22. The vertical axis plots frequency, where every division equals 30 MHz, and the horizontal axis plots amplitude, where each division equals l0 dB. It is seen that the only signal of substantial amplitude applied to the input of the first amplifier 22 is the fundamental signal (at 30 MHz) and that all harmonics as well as the D.C. component are less than -56 dBc. Referring now to Figure 8, it is seen that the output signal form the first inverting amplifier 22 comprises the fundamental signal (at 30 MHz), as well as even and odd order harmonies. (Note that the amplifier of Figure 5 is A.C. coupled and generates no D.C. output component. If the amplifier were D.C. coupled, a D.C. component would exist at the output of the first amplifier but would be largely suppressed at the output of the second.) This is due to the nonlinearities exhibited by the active bipolar semiconductor devices in the amplifier. The spectrum analyzer was set to plot harmonics up to 300 MHz. Obviously, the analyzer could have looked at harmonics having higher frequencies, but it was deemed not to be necessary in order to illustrate the operation of the present invention. Further, note that the bandwidth of the amplifier shown in Figure 5 above is .l MHz to 250 MHz. As discussed below, it will become apparent that the bandwidth of the amplifier will have a certain effect on the test results obtained with respect to the higher order harmonics. Figure 8 shows that the output from the first inverting amplifier 22 has even and odd-order harmonics that are of significant amplitude when compared to the 30 MHz fundamental signal. Specifically, note that the levels of the second and third order harmonics are -20 dBc as compared to the 30 MHz fundamental signal. Figure 9 plots the output of the attenuator l8, where the same horizontal and vertical axes of Figures 7 and 8 are used. Note that the level of the 30 MHz signal has been attenuated by approximately l2 dB as has the level of the even and odd harmonics. This result agrees with the theoretical results presented above since the attenuator is attenuating equally all of the components present at the output of the first amplifier 22. Thus, it is seen that the fundamental and the even and odd order harmonics present on line 20 have been attenuated by an equal amount by attenuator 26. Figure l0 plots the spectrum of the signals provided at the output of the second inverting amplifier 30. Again, the horizontal and vertical axes are the same as those of Figures 7-9. Figure l0 dramatically shows the reduction in the even-order harmonics produced by the present invention. Specifically, it is seen that the second order harmonic component is -4l dB below that of the fundamental output signal. It also shows that the fourth harmonic is -38 dB below that of the fundamental output signal. In contrast, the third harmonic signal is -l6 dB below the fundamental output signal. These test results corroborate the theoretical analysis presented above in subsection B. It should be understood that the values for the fifth through tenth harmonics are somewhat different from that predicted by the theoretical analysis due to the bandwidth of each of the inverting amplifiers of the test set-up. However, these higher order harmonics can be eliminated using low pass filters so as not to be of any significant amplitude in the output of the second inverting amplifier 30. Figure l0 graphically illustrates the improvement in the reduction of the second and fourth harmonic components produced by the present invention. It graphically corroborates the theoretical analysis presented above. Note that the third order harmonic has been increased by 4 dB, which is 2 dB less than the theory predicts. The fifth and higher order harmonics, as stated above, are not in substantial agreement with the theoretical results due to limitations in the actual circuitry that is employed to produce these test results. (The amplifiers used in the test set-up, while having a bandwidth of .l MHz to 250 MHz, did not exhibit such a response in actual practice.) It should be understood, however, that these higher order harmonic components can be attenuated significantly through the use of low pass filters and other suitable types of harmonic removal techniques,and are often of negligible amplitude compared to the 2nd and 3rd harmonies. Figure ll shows in block diagram form the present invention where a power divider has been substituted for a resistive attenuator. Note that the output from the power divider can be utilized by any type of utilization device. For example, this signal from the power divider, which is not needed by the present invention, can be utilized to control other circuit functions, such as a level control in a frequency synthesizer. In this way, this signal can also be used to perform other functions associated with other circuits with which the present invention can be used. It is also noted that the two inverting amplifiers can be fabricated to use the same substrate. When the active devices of such amplifiers use the same substrates, their characteristics can be made to track so that the amplifiers exhibit substantially identical operation. Other techniques can be used to obtain the desired identical operation of the two inverting amplifiers. The present invention can be used in any desired frequency range. It is particularly useful in the radio frequency (R.F.) and audio frequency (A.F.) ranges. While the present invention produces the desired attenuation of even-order harmonics when the gain, in dB, of each of the two inverting amplifier stages is equal to the attenuation loss, in dB, of the attenuator, it should be understood that situations can be imagined where the gain values are slightly different, albeit with less reduction expected in the D.C. and even-order harmonics.

Claims: Apparatus for amplifying an input signal (x i ), comprising: first inverting amplifier means (22) having an input adapted to be coupled to the input signal (x i ) for providing a first signal (x₁) at a first output which is based on the input signal (x i ); and second inverting amplifier means (30) having an input coupled to receive a signal derived from the first inverting amplifier means (22) for providing an output signal (x o ); characterized by: the first inverting amplifier means (22) having a predetermined, arbitrary gain -A; attenuator means (26) having an input coupled to said first output for providing a second signal (x₂) at a second output which is based on said first signal (x₁) attenuated by an attenuation factor 1/A substantially equal to said gain A; and the second inverting amplifier means (30) being substantially identical to the first inverting amplifier means (22) and having its input coupled to said second output for providing the output signal (x o ) at a third output which is based on said second signal (x₂) amplified by said predetermined gain -A. Apparatus as claimed in claim 1, characterized in that the first and second inverting amplifier means (22, 30) each include a first active device exhibiting a nonlinear amplifying characteristic. Apparatus as claimed in claim 1 or 2, characterized in that said attenuator means (26) comprises a resistive element. Apparatus as claimed in claim 1 or 2, characterized in that said attenuator means (26) comprises a power divider. Apparatus as claimed in any preceding claim, characterized in that said first inverting amplifier means (22) and said second inverting amplifier means (30) are fabricated using a portion of the same semiconductor substrate. Apparatus as claimed in any preceding claim, characterized in that said first and second inverting amplifier means (22, 30) each operate in the radio frequency range. Apparatus as claimed in any preceding claim, characterized by a utilization means connected to said attenuator means (26) to provide a function in accordance with said second signal. An amplifying method for substantially attenuating even-order harmonics, comprising the steps of: (a) amplifying and inverting an input signal (x i ) in amplifier means (22) to produce a first signal (x₁); and (c) amplifying and inverting a signal (x₂) derived from said amplifier means (22) in a further amplifier means (30) to produce an output signal (x o ); characterized by: the amplifier means (22) of step (a) having a predetermined arbitrary gain -A; a step (b), performed between steps (a) and (c), of attenuating said first signal (x₁) by an attenuation factor 1/A substantially equal to the said gain A to produce a second signal (x₂); and the further amplifier means (30) of step (c) being substantially identical to said amplifier means (22) and amplifying and inverting said second signal (x₂) by said predetermined gain -A.

Inventors: BOTTMAN, JEFFREY S.
Applicants: JOHN FLUKE MFG. CO., INC.
Application year: 1985
Main classification: H03F1
IPC codes: H03F1, H03F3