Abstract:
Constant and accurate signal gain systems based on controlling oscillator loop gain. A constant gain positive feedback network and an amplifier form an oscillator. Only when the oscillator loop gain is at least one does the oscillator produces an AC signal. Negative feedback of the oscillator&#39;s AC signal level is used to keep the loop gain close to or at the value of one by controlling the loop gain of the oscillator circuit. By maintaining the loop gain of the oscillator circuit substantially constant the signal gain is also maintained substantially constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The benefits of filing this invention as Provisional application for patent “Constant gain amplifier system with positive and negative feedback”, U.S. PTO #60/931,907 filed May 26, 2007 by Fred Mirow are claimed. 

   BACKGROUND OF THE INVENTION 
   This invention relates to systems for providing a constant and accurate signal gain with relatively wide bandwidth based on controlled oscillator loop gain and is described. The constant gain positive feedback network combined with an amplifier forms an oscillator. Only when the oscillator loop gain is at least one does the oscillator produces an AC signal. Negative feedback of the oscillator&#39;s AC signal level is used to control the loop gain of the oscillator circuit, keeping the loop gain close to or at one. By maintaining the loop gain of the oscillator circuit substantially constant, the signal gain of the amplifier is maintained substantially constant with the gain primarily set by the positive feedback network gain and to a lesser extent the gain of the negative feedback loop. 
   In general, the prior art systems have used negative feedback between the output and input of the amplifying circuit for obtaining an accurate constant signal gain. A major problem in such systems has been that to maintain amplifier stability the amplifier frequency bandwidth needed to be limited to obtain relatively large gain and phase margins. 
   The present invention overcomes the bandwidth limitation of the negative feedback amplifying circuit by allowing the system to become unstable and oscillate. Thus the present invention does not have the same limited frequency bandwidth requirements to maintain large gain and phase margins. 
   An objective of the present invention is to provide a constant and accurate signal gain system that has a high temperature, radiation, and voltage stability due to its reliance on passive component ratios to set circuit gain values. Passive components such as resistors and capacitors are more stable under these conditions. This invention increases system accuracy by making the accuracy dependent on passive component ratios instead of transistor stability. 
   Another object of the invention is to provide bandwidth improvement in constant gain signal amplifiers. 
   A further objective of the invention to provide circuits that are less susceptible to process variances by relying on impedance ratios thereby providing a more consistently manufacturable circuit. 
   BRIEF SUMMARY OF THE INVENTION 
   According to this invention, a feedback network section combined with an amplifier section are used along with other elements to form a oscillator. The preferred frequency of oscillation is a frequency that is separated from the input signal frequency range by an amount that makes separation of the two signal relatively easy. In the case when the system only oscillates when the input signal is not being amplified, the frequency of oscillation can also be a frequency that is the same as the input signal frequency. 
   At the preferred frequency of oscillation, the amplifier has a certain phase shift and the feedback network supplies the remaining phase shift necessary to make the total phase shift at the frequency of oscillation 0 degrees. There are many well known phase shift networks that can be used as part of feedback network such as the twin T, inductor capacitor (LC), and the Wien bridge. 
   This oscillator will only oscillate when the loop gain of the amplifier and feedback network is at least one when the phase shift is zero degrees. The oscillator state can readily be detected. When oscillating a oscillator AC signal is present and when not oscillating, no AC signal is present at the oscillation frequency. The frequency stability of the oscillation is not critical and it can vary with changes in temperature, voltage, etc. within a relatively wide range that is taken into account during system design, for example 15 percent. 
   The feedback network uses circuits that depend on the ratio of circuit components, preferably these are passive devices such as capacitors, inductors, and resistors, to provide stable gain over temperature. 
   The amplifier whose gain is to be maintained at a selected constant value is connected to the feedback network so as form an oscillator. The input signal that is to be amplified is added to the feedback network signal and applied to the amplifier input. The feedback network blocks (significantly reduces the signal amplitude) the amplified input signal, allowing only the oscillation AC signal to pass (not significantly reducing the signal amplitude) through. The amplifier output contains the amplified input signal along with the oscillation AC signal. An other filter blocks the oscillation AC signal, allowing only the amplified input signal to pass through to the systems output. 
   The amplifier gain is kept constant by using negative feedback of the oscillator AC signal level to control the amplifier gain. As the amplifier gain is varied so is the oscillator&#39;s loop gain. The oscillator AC signal level controls a gain varying element of the amplifier. As the oscillator AC signal level varies the loop gain varies. In some cases it may be more convenient to add a separate variable gain network before or after the amplifier to control the signal gain instead of directly varying the amplifier gain. The oscillator AC signal level is of a value that just maintains oscillation by keeping the loop gain at or close to one. Since the gain of oscillator loop, excluding the amplifier, is substantially constant the negative feedback of the oscillator AC signal level to control the amplifier gain maintains the amplifier gain also substantially constant. 
   In some systems the input signal is applied to a low gain amplifier or even a less than unity gain network such as an optical coupler or laser system. In this situation the positive feedback loop gain may not be enough for oscillation to occur. A fixed gain amplifier is inserted into the positive feedback loop to provide the additional gain needed for oscillation. 
   This fixed gain amplifier has a limited frequency response requirement due to the fact it only needs to amplify at the frequency of oscillation. Negative feedback can be used on this amplifier. One well know approach is to use a high gain amplifier which uses negative feedback to accurately set the amplifier section&#39;s gain. The negative feedback is optimally obtained by using a resistor divider network in which the resistor temperature and voltage characteristics are matched. The effects of temperature on the resistor network are then decreased. This also decreases the effects on the amplifier section since the gain of it is primarily determined by the resistor divider network. 
   There are many well known methods for implementing amplifier gain varying elements. One method is to use signal controlled variable impedance devices. Some examples for use with voltage control signals are variable resistors obtained by using a FET with it&#39;s gate receiving the control signal. Likewise there are many well known methods for obtaining signal controlled variable gain devices. An example of this are transistors. The transistor&#39;s gain is varied by varying it&#39;s DC operating current level in response to the control signal thus obtaining a signal controlled variable gain device. Also, the inductive coupling (mutual inductance) between inductors may be changed in response to the control signal. 
   In some systems the amplifier&#39;s input offset voltage also needs to be cancelled along with maintaining constant signal gain. This is accomplished by adding well known offset cancellation techniques to the systems for providing a constant and accurate signal gain. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments. 
       FIG. 1  shows a block diagram of constant gain amplifier system  1 ; 
       FIG. 2  shows a block diagram of constant gain network system  2 ; 
       FIG. 3  shows a block diagram of constant gain network system  3 ; 
       FIG. 4  shows a block diagram of constant gain amplifier system  4 ; 
       FIG. 5  shows a block diagram of constant gain amplifier system  5 ; 
       FIG. 6  shows a block diagram of constant gain and amplifier offset voltage auto zero system  6 ; 
       FIG. 7  shows a block diagram of constant gain and amplifier offset voltage auto zero system  6 A; and 
       FIG. 8  shows a block diagram of constant gain amplifier in parallel with an identical amplifier receiving the same gain control signal. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the invention is shown in  FIG. 1 . The constant gain amplifier system  1  comprising network  9 , detector  7 , signal combiner  13 , amplifier  8 , filter  12 , input terminal  20  and output terminal  21 . The output of network  9  on line  15  is connected to the input of detector  7  and also to a input to signal combiner  13 . The other input of signal combiner  13  is connected to input terminal  20 . Signal combiner  13  provides a output signal that is the sum of it&#39;s two input signals on line  16  to the input of amplifier  8 . Amplifier  8  receives signals to it&#39;s gain control input from the output of detector  7  on line  14 . The output of amplifier  8  is on line  6  which is connected to the inputs of filter  12  and network  9 . 
   Network  9 , signal combiner  13 , amplifier  8 , form a positive feedback loop which oscillates. The signal path between input terminal  20  and output terminal  21  is through signal combiner  13 , amplifier  8 , and filter  12 . 
   Amplifier  8  amplifies the signal applied to it&#39;s input with a gain controlled by the signal level at it&#39;s gain control input. The bandwidth of amplifier  8  is wide enough to provide uniform gain across the entire input signal frequency range and also the frequency of oscillation. Filter  12  blocks (significantly reduces the signal amplitude) the oscillator AC signal from being applied to output terminal  21  while passing (not significantly reducing the signal amplitude) the amplified input  20  signal. Filter  12  and network  9  have a constant or high enough input impedance to not significantly vary the gain of amplifier  8  across the frequency range of interest. Network  9  provides phase shift dependant on frequency and substantially blocks the input signal frequencies. The network  9  phase shift is selected to provide the necessary amount of phase shift along with that of amplifier  8  and signal combiner  13  to ensure oscillation occurs within the selected frequency range. The level of detector  7  gain control output signal is proportional to the oscillation AC signal level at it&#39;s input on line  15 . Detector  7  also converts the signal at it&#39;s input to the type of output signal desired at the amplifier  8  gain control input. 
   The signal gain of network  9 , detector  7 , signal combiner  13 ,and filter  12  are substantially constant. The gain of amplifier  8  varies as the oscillator AC signal level of line  15  varies. As the oscillator AC signal level of line  15  increases, the gain of amplifier  8  decreases causing the oscillator loop gain to be reduced. When the oscillator loop gain is reduced below one, the oscillation stops, and the oscillator AC signal level on line  15  becomes zero. Thus the detector  7  output signal level is of a value that just maintains oscillation. In effect detector  7  provides negative feedback to just maintain a low oscillator AC signal level on line  15  which occurs at a oscillator loop gain at or close to one. Since the only circuit section effecting oscillator loop gain that is not substantially constant is that of amplifier  8 , the negative feedback through detector  7  maintains the gain of amplifier  8  substantially constant. The value of amplifier  8  gain is primarily set by the gain of the rest of the positive feedback loop with the gain of the negative feedback loop through detector  7  having a lesser effect. For example by reducing the network  9  gain in half the gain of amplifier  8  is substantially doubled because the product of the gain around the oscillator loop is kept at or close to one. 
   An other embodiment of the invention is shown in  FIG. 2 . The constant gain network system  2  comprising network  9 , amplifier  10 , detector  7 , signal combiner  13 , network  11 , filter  12 , input terminal  20  and output terminal  21 . The output of amplifier  10  on line  17  is connected to the input of detector  7  and one of the signal combiner  13  inputs. The other input of signal combiner  13  is connected to input terminal  20 . Signal combiner  13  provides a output signal that is the sum of it&#39;s two input signals on line  18  to the input of network  11 . Network  11  receives signals to it&#39;s gain control input from the output of detector  7  on line  14 . The output network  11  is on line  6  which is connected to the inputs of filter  12  and network  9 . 
   Network  9 , amplifier  10 , signal combiner  13 , and network  11 , form a positive feedback loop which oscillates. The signal path between input terminal  20  and output terminal  21  is through signal combiner  13 , network  11 , and filter  12 . 
   Network  11  gain is controlled by the signal level at it&#39;s gain control input. Network  11  may be a passive network with less than unity signal gain or an active network with low signal gain. Constant gain amplifier  10  provides the additional gain needed for oscillation. The bandwidth of network  11  is wide enough to provide uniform gain across the entire input frequency range and also the frequency of oscillation. Filter  12  blocks the oscillator AC signal from being applied to output terminal  21  while passing the input  20  signal frequencies. The network  9  phase shift is selected to provide the necessary amount of phase shift along with that of amplifier  10 , signal combiner  13 , and network  11  to ensure oscillation occurs in the selected frequency range. Detector  7  converts the oscillator AC signal at it&#39;s input on line  17  to the type of output signal desired at the network  11  gain control input. 
   The signal gain of network  9 , detector  7 , amplifier  10 , signal combiner  13 , and filter  12  are substantially constant. The gain of network  11  varies as the oscillator AC signal level of line  17  varies. As the AC signal level of line  17  increases, the gain of network  11  decreases causing the oscillator loop gain to be reduced. When the oscillator loop gain is reduced below one, the oscillation stops, and the oscillator AC signal level on line  17  becomes zero. Thus the detector  7  output signal level is of a value that just maintains oscillation. In effect detector  7  provides negative feedback to maintain a low AC signal level on line  17  which occurs at a oscillator loop gain at or close to one. Since the only oscillator loop gain section in which the gain is not substantially constant is that of network  11 , the negative feedback through detector  7  maintains the gain of network  11  at a substantially constant value. The value of network  11  gain is primarily set by the gain of the rest of the positive feedback loop. For example by reducing the network  9  gain in half the gain of network  11  is substantially doubled because the product of the gain around the oscillator loop is kept at or close to one. 
   An other embodiment of the invention is shown in  FIG. 3 . The constant gain network system  3  comprising network  9 , amplifier  10 , detector  7 , filter  22 , filter  23 , network  11 , filter  12 , input terminal  20  and output terminal  21 . The output of amplifier  10  on line  17  is connected to the input of detector  7  and filter  22 . The input of filter  23  is connected to input terminal  20 . The output of filter  22  and filter  23  on line  24  is a signal that is the sum of both filters  22  and  23  input signals. Network  11  receives signals to it&#39;s gain control input from the output of detector  7  on line  14 . The output of network  11  is on line  6  which is connected to the inputs of filter  12  and network  9 . 
   Network  9 , amplifier  10 , filter  22 , and network  11 , form a positive feedback loop which oscillates. The signal path between input terminal  20  and output terminal  21  is through filter  23 , network  11 , and filter  12 . Filter  23  blocks the oscillator AC signal on line  24  signal flow to input terminal  20 . Filter  22  passes the oscillator AC signal and blocks the input signal frequencies from the input of detector  7 . Filter  12  blocks the oscillator AC signal from being applied to output terminal  21  while passing the input  20  signal frequencies. Filters  12 ,  22 , and  24  have a constant or high enough input impedance to not significantly vary the gain of amplifier  8  across the frequency range of interest. 
   The network  9  phase shift is selected to provide the necessary amount of phase shift along with that of amplifier  10 , filter  22 , and network  11  to ensure oscillation occur in the selected frequency range. The output signal level of detector  7  on line  14  is proportional to the oscillation AC signal level at the output of amplifier  10 . 
   The signal gain of network  9 , amplifier  10 , filter  22 , filter  23  and filter  12  are substantially constant. The network  11  gain varies as the oscillator AC signal level at the output of amplifier  10  varies. As the oscillator AC signal level of at the output of amplifier  10  increases, the gain of network  11  decreases causing the oscillator loop gain to be reduced. When the oscillator loop gain is reduced below one, the oscillation stops, and the oscillator AC signal level at the output of amplifier  10  becomes zero. Thus the detector  7  output signal level is of a value that just maintains oscillation. In effect detector  7  provides negative feedback to maintain a low AC signal level at the output of amplifier  10  which occurs at a oscillator loop gain at or close to one. Since the only oscillator loop gain section in which the gain is not substantially constant is that of network  11 , the negative feedback through detector  7  maintains the gain of network  11  substantially constant. The network  11  gain is primarily set by the gain of the rest of the positive feedback loop. 
   An other embodiment of the invention is shown in  FIG. 4 . The constant gain amplifier system  4  comprising amplifier network  90 , controller  32 , input terminal  20  and output terminal  21 . Amplifier network  90  consist of network  9 , memory  39 , relay  35 , relay  36 , and amplifier  8 . 
   The input signal level at input terminal  33  controls the controller  32  output signal resulting in either a high or low level on line  31  or alternatively controller  32  may have an internal oscillator causing repeated cycling of output line  31  between high and low level independently of the input signal at input terminal  33 . Line  31  is connected to relay  35 , relay  36 , and memory  39 . 
   When the line  31  signal is high level, constant gain system  4  is in the oscillate mode. Network  9 , and amplifier  8 , form a positive feedback loop which oscillates when relay  35  connects the amplifier  8  input to line  37  and relay  36  connects the amplifier  8  output to line  38 . The output of network  9  on line  37  is also connected to a input of memory  39 . 
   The bandwidth of amplifier  8  is wide enough to provide uniform gain across the entire input frequency range and also the frequency of oscillation. The signal gain of network  9 , and memory  39  are substantially constant. Network  9  provides phase shift dependant on frequency. The network  9  phase shift is selected to provide the necessary amount of phase shift along with that of amplifier  8  to ensure that oscillation occur in the selected frequency range. The oscillation frequency may be the same as the input signal frequency since the input signal is not connected at the same time the amplifier network  90  is in oscillate mode. 
   The output signal level of memory  39  is proportional to the oscillation AC signal level on line  37 . Memory  39  converts the signal on line  37  to the type of output signal on line  34  desired at the amplifier  8  gain control input. Amplifier  8  amplifies the signal level applied to it&#39;s input by a gain controlled by the signal level on line  34  which is connected to it&#39;s gain control input. The gain of amplifier  8  decreases as the oscillator AC signal level of line  37  increases. When the oscillator loop gain is reduced below one, the oscillation stops, and the oscillator AC signal level on line  37  becomes zero. Thus the memory  39  output signal level is of a value that just maintains oscillation. In effect memory  39  provides negative feedback to maintain a low AC signal level on line  37  which occurs at a oscillator loop gain at or close to one. Since the only gain that is not substantially constant is that of amplifier  8 , the negative feedback through memory  39  maintains the gain of amplifier  8  substantially constant. The amplifier  8  gain is primarily set by the gain of the rest of the positive feedback loop. For example by reducing the network  9  gain in half the gain of amplifier  8  is substantially doubled because the product of the gain around the oscillator loop is kept at or close to one. 
   When line  31  is at low level, amplifier network  90  is in the amplify mode. The signal flow is through input terminal  20 , amplifier  8 , to output terminal  21  with relay  35  connecting the amplifier  8  input to input terminal  20  and relay  36  connecting the amplifier  8  output to output terminal  21 . The output signal of memory  39  remains constant at the last value of the signal level on line  34  during oscillate mode. The gain of amplifier  8  is maintained substantially constant by the stored output signal of memory  39  until the next time the amplifier  8  is readjusted during oscillate mode. 
   An other embodiment of the invention is shown in  FIG. 5 . The constant gain amplifier system  5  comprising amplifier network  90 A, amplifier network  90 B, controller  48 , inverter  47 , input terminal  20  and output terminal  21 . 
   Controller  48  has an internal oscillator causing repeated cycling of output line  41  signal state between high and low levels. Line  41  is connected to the control input of amplifier network  90 B and inverter  47 . The inverter  47  output signal on line  42  is the opposite level of that on line  41 . Line  42  is connected to the control input of amplifier network  90 A. 
   When the signal level on line  41  is high, amplifier network  90 B is in the oscillate mode and amplifier network  90 A is in the amplify mode. When the signal level on line  41  is low, amplifier network  90 B is in the amplify mode and amplifier network  90 A is in the oscillate mode. 
   The signal flow is through input terminal  20  and which ever amplifier network  90 A or  90 B, is in the amplify mode to output terminal  21  while the other amplifier network, either  90 A or  90 B, which is in the oscillate mode has it&#39;s signal gain readjusted to the predetermined value. This allows continuous amplification of the signal applied to input terminal  20  and also the gain to be periodically readjusted to the correct value without interrupting signal flow through system  5 . 
   An other embodiment of the invention is constant gain and amplifier offset voltage auto zero system  6  shown in  FIG. 6 . This system  6  comprising controller  32 , network  9 B, memory  39 , relay  35 , relay  36 , and amplifier  8 A, capacitor  83 , reference  96 , input terminal  20 , and output terminal  21 . 
   The input signal at input terminal  33  controls the controller  32  output signal resulting in either a high or low level on line  31  or alternatively controller  32  may have an internal oscillator causing repeated cycling of output line  31  between high and low level independently of the input signal at input terminal  33 . Line  31  is connected to relay  35 , relay  36 , and memory  39 . 
   When the line  31  signal is at the high level, constant gain and amplifier offset voltage auto zero system  6  is in the oscillate and amplifier offset voltage auto zero mode. Amplifier  8 A is a differential input amplifier. Network  9 B, capacitor  83 , and amplifier  8 A, form a positive feedback loop which oscillates when relay  35  connects one amplifier  8 A input to line  37  and relay  36  connects the amplifier  8 A output through capacitor  83  to line  38 . The other amplifier  8 A input is connected to voltage reference  96 . The output of network  9 B on line  37  is also connected to the input of memory  39 . 
   Network  9 B has a low DC impedance level between line  37  and voltage reference  96 , and also line  38  to ground. Voltage reference  96  may be set to any desired voltage level including zero volts (ground). Network  9 B blocks DC voltage between lines  37  and  38  unless of course they are at the same voltage level. 
   With substantially zero DC voltage difference applied between the two inputs of amplifier  8 A, the output of amplifier  8 A on line  84  is substantially at a DC voltage level equal to it&#39;s gain times it&#39;s input offset voltage. The capacitor  83  is connected to line  84  and the other side through relay  36  and the input of network  9 B to ground, it is understood that an other voltage level besides ground could also be used. This charges capacitor  83  to the DC voltage level difference between line  84  and ground. The oscillation AC voltage and the input offset voltage have substantially no effect on each other since the AC voltage level is of low amplitude and substantially sinusoidal. 
   When the line  31  signal is in the low state, constant gain and amplifier offset voltage auto zero system  6  is in the oscillate and amplifier offset voltage auto zero mode. The signal flow is into input terminal  20 , through amplifier  8 A, and capacitor  83  to output terminal  21  with relay  35  connecting the amplifier  8 A input to input terminal  20  and relay  36  connecting capacitor  83  to output terminal  21 . The DC voltage level on capacitor  83  is now added to the voltage level on line  84  resulting in cancellation of the DC input offset voltage at output terminal  21 . Output terminal  21  needs to be connected to a high external load resistance to prevent a significant discharge of the voltage stored on capacitor  83 . 
   The output signal level of memory  39  remains constant at the last value of the signal level on line  34  during oscillate and amplifier offset voltage auto zero mode. The gain of amplifier  8 A is maintained at a substantially constant value by the stored output signal of memory  39  and it&#39;s DC input offset voltage remains substantially cancelled by the stored voltage on capacitor  83  until the next time constant gain and amplifier offset voltage auto zero system  6  is readjusted during oscillate and amplifier offset voltage auto zero mode. 
   An other version of the invention constant gain and amplifier offset voltage auto zero system  6  is shown in  FIG. 7  as constant gain and amplifier offset voltage auto zero system  6 A. This constant gain and amplifier offset voltage auto zero system  6 A comprising controller  32 , memory  39 , relay  35 , relay  36 , amplifier  8 A, capacitor  83 , network  9 C, input terminal  20  and output terminal  21 . Network  9 C consist of capacitor  95 , resistor  93 , and resistor  94 . 
   When line  31  is at the high level, constant gain and amplifier offset voltage auto zero system  6 A is in the oscillate and amplifier offset voltage auto zero mode. Network  9 C, capacitor  83 , and amplifier  8 A, form a positive feedback loop which oscillates when relay  35  connects the amplifier  8 A input to line  37  and relay  36  connects capacitor  83  to line  38 . 
   The output of network  9 C on line  37  is also connected to the input of memory  39 . In network  9 C resistor  93  connects to line  38  and it&#39;s other side to capacitor  95 , resistor  94 , and line  37 . Capacitor  95  and resistor  94  both have their other side connected to ground. 
   The resistors of network  9 C, resistor  94  and resistor  93 , create a relatively low DC impedance level to ground having substantially zero DC voltage at network  9 C output on line  37 . The capacitor  83  in combination with the elements of network  9 C form a phase shift network. This phase shift network when connected to the non-inverting input of amplifier  8 A forms an oscillator. 
   When line  31  is at the level low, constant gain and amplifier offset voltage auto zero system  6 A is in the amplify mode. The output signal level of memory  39  remains constant at the last value of the signal level on line  34  during oscillate mode. The gain of amplifier  8 A is maintained at a substantially constant value by the stored output signal of memory  39  and it&#39;s DC input offset voltage remains substantially cancelled by the stored voltage on capacitor  83  until the next time constant gain and amplifier offset voltage auto zero system  6 A is readjusted during the oscillate and amplifier offset voltage auto zero mode. 
   An other approach to obtaining a constant gain amplifier system is two use two substantially identical circuits which maybe either amplifiers or networks. One of the identical circuits is used in the input signal path while the other identical circuits is used as a section of the oscillator loop. Both identical circuits receive the same gain control signal. In the case of using amplifiers one of the amplifiers is used to amplify the input signal while the other amplifier is used as a section of the oscillator loop. The same gain control signal that is applied to the oscillator loop amplifier is also applied to the amplifier connected to the input signal. 
   An embodiment of this approach using amplifiers is shown in  FIG. 8 . The constant gain amplifier system  7  consist of the oscillator section and the amplifier section. 
   The oscillator section comprising network  9 , detector  7 , and amplifier  8 . The output of network  9  on line  15  is connected to the input of amplifier  8  and detector  7 . Amplifier  8  receives the signal to it&#39;s gain control input from the output of detector  7  on line  14 . The output of amplifier  8  is on line  6  which is connected to the input and network  9 . Network  9 , amplifier  8 , form a positive feedback loop which oscillates. 
   The amplifier section comprising amplifier  108 , input terminal  20  and output terminal  21 . The signal flow is through input terminal  20 , then amplifier  108 , to output terminal  21 . Amplifier  108  receives the signal to it&#39;s gain control input from the output of detector  7  on line  14 . 
   Amplifier  8  and  108  amplify the signals applied to their inputs by a gain value controlled by the signal level at their gain control input on line  14 . The bandwidth of amplifier  8  and  108  are wide enough to provide uniform gain across the entire input signal frequency range and also the frequency of oscillation. The impedance connected to the Amplifier  8  and  108  outputs should remain constant and preferably at the same value. The output signal level of detector  7  is proportional to the oscillation AC signal level at it&#39;s input on line  15 . Detector  7  also converts the signal at it&#39;s input to the type of output signal desired at the amplifier  8  and  108  gain control inputs. 
   The gain of amplifier  8  varies as the oscillator AC signal level of line  15  varies. As the oscillator AC signal level of line  15  increases, the gain of amplifier  8  decreases causing the oscillator loop gain to be reduced. When the oscillator loop gain is reduced below one, the oscillation stops, and the oscillator AC signal level on line  15  becomes zero. Thus the detector  7  output signal level is of a value that just maintains oscillation. In effect detector  7  provides negative feedback to just maintain a low oscillator AC signal level on line  15  which occurs at a oscillator loop gain at or close to one. Since the only oscillator loop gain that is not substantially constant is that of amplifier  8 , the negative feedback through detector  7  maintains the gain of amplifier  8  at a substantially constant value. The value of amplifier  8  gain is primarily set by the gain of the rest of the positive feedback loop with the gain of the negative feedback loop through detector  7  having a lesser effect. Since amplifier  8  and  108  are substantially identical by maintaining amplifier  8  gain constant amplifier  108  gain is also kept constant. 
   Although the above description has been directed to preferred embodiments of the invention, it will be understood and appreciated by those skilled in the art that other variations and modifications may be made without departing from the spirit and scope of the invention, and therefore the invention includes the full range of equivalents of the features and aspects set forth in the appended claims.