AMPLIFIER

Abstract: An amplifier as used in a tuner of video and/or audio equipment has to amplify the received input signal but should add as little as possible noise and distortion to the amplified signal. To improve the operation of the amplifier and of the tuner, the amplifier of the invention comprises at least two gain stages and switching means to short-circuit a gain stage when it is not necessary for the overall gain value.

Description: Amplifier. The invention relates to a tuner as claimed in the preamble of claim 1. The invention also relates to a multistage amplifier, for example, for use in a tuner as an LF amplifier, a video LF amplifier, an audio LF amplifier, a video or audio baseband amplifier. The invention also relates to a cascaded current comparator for use in a multistage amplifier. Such a tuner is known from US-A-5,210,504. Herein a tuner is described for satellite broadcasting. Such a tuner is also known from the article "Monolithic TV IF system TBA 440", E. Schatter, IEEE Transactions on BTR, Vol. 18, august 1972, pages 158-163. In the above-mentioned article, an (LF) amplifier has been described comprising a series arrangement of three gain stages. A disadvantage of the prior-art tuner is that such a tuner adds noise and distortion to the input signal, especially at a low gain with several gain stages in series. An object of the invention is to overcome the disadvantages of the prior-art tuner. To this end, a first aspect of the invention provides a tuner as defined in claim 1. A second aspect of the invention provides a multistage amplifier as defined in claim 4. By shunting the unnecessary gain stage/stages the problem of the tuners and/or amplifiers of the prior art are overcome. A third aspect of the invention provides a cascaded current comparator as defined in claim 6. By turning off inactive stages, current dissipation is reduced resulting in a low-power cascaded current comparator. An embodiment of a tuner according to the invention comprises the features of claim 2. By using three gain stages the necessary gain range can be easily obtained, and at the same time the complexity of the amplifier is kept within reasonable proportions. An embodiment of a tuner according to the invention comprises the features of claim 3. By short-circuiting at least two gain stages together for the low gain range the distortion is further reduced. An embodiment of a multistage amplifier according to the invention comprises the features of claim 5. In this way a so-called thermometer code is obtained for supplying the tuning voltages of the gain stages. The invention and additional features, which may optionally be used to implement the invention to advantage, will be apparent from and elucidated with reference to the examples described hereinafter. In the drawings: Figure 1 shows a schematic embodiment of a tuner according to the invention, Figure 2 shows a schematic embodiment of an IF amplifier according to the invention, Figure 3 graphically shows the overall gain versus the gain control quantity, Figure 4 shows an embodiment of a circuit of a gain stage, Figure 5 shows an embodiment of a switch, and Figure 6 shows an embodiment of a comparator. Figure 1 shows, by way of example, a schematic embodiment of a tuner T according to the invention, comprising an input I and an output O. The input is coupled to an input amplifier 1, for amplifying the received antenna signal sa. The output of the input amplifier is coupled to a mixer 3 for obtaining a so-called IF signal. This IF signal, after filtering in a filter 5, is supplied to an LF amplifier 7 as a multistage amplifier. After amplification in the LF amplifier, the LF signal is further supplied to a second mixer 9 for obtaining a base band video-signal sv at the output O. The video-signal sv is supplied to control means 11 for controlling the amplification of the input amplifier 1 and of the LF amplifier 7 in this example. Figure 2 shows, by way of example, a schematic embodiment of a multi stage amplifier 27. In this embodiment the multistage amplifier comprises three gain stages 271, 272 and 273. Further, the LF amplifier comprises switching means comprising a switch SW2 parallel to the second gain stage 272 and a switch SW3 parallel to the second gain stage 272 and the third gain stage 273. Alternatively, switch SW3' could be shunting only the third gain stage 273. It is to be noted here that the multistage amplifier 27 can be an LF amplifier (see figure 1, reference numeral 7), or a baseband amplifier. The multistage amplifier can further be used for video or audio signals. For strong antenna signals the gain of the multistage amplifier, in this embodiment, is equal to 0.5 (-6dB) whereas for weak signals the gain can increase up to 2000 (66dB). The large gain range is established in this example by three cascaded gain stages. Every gain stage adds noise and distortion to the output signal, which lowers the overall dynamic range at low gain values. However, at low gain values only one gain stage is needed for the amplification and the other two can be shunted. In this concept gain stages are shunted if they are not needed for the amplification resulting in a higher dynamic range. Furthermore, by a proper way of controlling the gain stages the distortion requirements for the intermediate gain stage(s) can be relaxed significantly. In order to establish a certain dynamic range and keep the power as low as possible, noise of the amplifier must be as low as possible. In the following, the output noise will be analysed. Let us assume that the switches SW2 and SW3 are open; then the square mean output noise voltage is given by: v; oω = (G\ * G2 * G3) * v 2 271 + (G2 * G3) 2 * v 2 272 + (G3) 2 v 2 273 Wherein Gl, G2 and G3 is the gain of stage 271, 272 and 273, respectively. The noise voltage of the gain stages is referred to the input. If the gain of 271, 272 and 273 is low, for example G1=G2=G3=1, and all three stages are identical, the noise contribution to the output noise is the same for every stage. However, in this case, the stage 272 and 273 are not needed for the amplification. Consequently, shunting the stages 272 and 273 will lower the output noise and does not affect the overall gain since G2=G3=1. This relaxes the noise requirements for all three stages significantly. For a given dynamic range, stage 271 is allowed to produce more noise since stage 272 and 273 do not contribute to the output noise. Moreover, the noise power of the stage 272 and 273 might increase as long as the noise power is negligible compared to G 2 * V 2 „271 resulting in the same overall output noise. Because the shunt switches won't be open as long as 271 has not reached its maximum gain, the higher allowable output noise of the stages can result in a lower power consumption. If the shunt switches SW2 and SW3 are closed, not only the noise is reduced but also the distortion due to stages 272 and 273. It is clear that stage 273 has to handle the maximum output voltage, however, this is not obvious for stage 272. If the gain of stage 273 increases before the gain of stage 272, the maximum output signal of stage 272, as stage 272 is open, will be reduced by a factor G3max. As a result of this, the distortion requirement for stage 272 has been relaxed significantly. Figure 3 shows the establishment of the overall gain versus the gain control quantity. This quantity is a voltage or a current whose value is determined by the control means. For low gain, only stage 271 is active and the shunt switches are closed. Just before stage 273 is activated, switch SW3 opens. At this point, the voltage across switch SW3 is ideally zero since the gain of stage 273 is equal to 1 and switch SW2 is closed. So the output signal will not be distributed due to switching of switch SW3. If stage 273 is close to its maximum value G3max, switch SW2 opens and stage 272 will increase the overall gain. During switching of switch SW2 the output signal will not be disturbed since G2=l. Although the LF amplifier in this embodiment has one intermediate stage, in general more intermediate stages can be applied. The requirements for the first stage are relatively high in this embodiment. Nevertheless, they are lower than the requirements in the situation wherein no switches are used. Most of the power will be consumed by the first stage. The intermediate stage on the other hand can be a very relaxed design with respect to the noise and linearity. Figure 4 shows an example of the circuit diagram of a gain stage 47x. All gain stages are based on the same principle. It consists of a resistor degenerated differential pair terminated with resistors. The differential input voltage (Vin, -Vin) is supplied via transistor Tla, Tib respectively. The differential output voltage (Vout, -Vout) is supplied at the connection point of transistor Tla, T2a; Tib, T2b respectively. Both the degeneration resistance (Rla, R2a, R3a; Rib, R2b, R3b) and the termination resistance (R4a, R5a, R6a; R4b, R5b, R6b) are adjustable with MOS transistors. In numerical order, the tune voltages Vtune (Vtunel, Vtune2, Vtune3) vary gradually from a low potential (0V) to a high potential (4V). In this way the MOS transistors are 'soft-switched' and the gain increases gradually. Alternatively, all the tune voltages can be connected to one node; in this way the MOS transistors can still be 'soft-switched' in the correct order due to the different source (or drain) voltages. An advantage of this gain stage is the relatively low noise performance. The differential mean square noise voltage of the degeneration or termination network is equal to 4*k*T*Reffdf, where Reff is the effective resistance of the network. Such a gain stage has been described in EP-A-0836766, in the name of the same applicant (PHN15802). This patent application and corresponding applications are incorporated herein by reference. For the stages 272 and 273, the gain is preferably exactly equal to 1 if the shunt switches are closed. For this reason, transistors T2a and T2b are added, in this example, to compensate for the finite transductance of Tla and Tib as well as for the distortion and temperature variations. Because of the different requirements with respect to noise and distortion, the gain stages can be individually optimised. The degeneration and termination resistance of the first stage is determined by the noise requirements. The total resistance of the second and the third stage is determined by the frequency behavior. The resistance of these stages might increase with respect to the noise requirements but it will result in a lower cut-off frequency. The number of applied 'soft-switches' is determined by the distortion requirements. In the first stage for example eleven 'soft-switches' are used whereas in the second stage for example only two 'soft-switches' are needed (in EP-A-0836766 this has been described in more detail). The applied shunt switch consists basically of a NMOS transistor. As mentioned before, the voltage drop across a shunt switch must be as small as possible to keep the distortion low during switching. Therefore, the gain of the second (272) and the third (273) stage must be preferably very close to 1 and the phase-shift must be preferably close to zero. However, even if the voltage drop across the switch is zero, the switch still introduces distortion due to the non-linear gate-source capacitance, which is the worst if Vgs is close to the Vt . To reduce the signal voltage across the^ate-source, the gate is coupled by a capacitor to the source. Figure 5 shows a simplified circuit diagram of a (shunt-) switch SW5x comprising a MOS transistor M51, a capacitor C51 and a resistor R51, controlled by means of a control signal Vctrl. The terminals of the main current path of the MOS transistor M51 are denoted by Vin and Vout. In the embodiment shown in Fig. 2, the shunt switch SW2 is connected with its Vin terminal to the input of the gain stage 272, and with its Vout terminal to the output of the gain stage 272. In a same way, the shunt switches SW3 and SW3' are arranged between the inputs of the series arrangement of the gain stages 272 and 273, and the gain stage 273, respectively. In the embodiment of a gain stage as shown in Fig. 4, two shunt switches are required. A first shunt switch receives the input voltage +Vin on its Vin terminal and is connected with its output terminal Vout to the node supplying +Vout. A second shunt switch receives the input voltage -Vin on its Vin terminal and is connected with its output terminal Vout to the node supplying -Vout. Special provisions may be needed to ensure the proper DC levels of the input and output voltages +Vin, -Vin, +Vout and -Vout. If the tune voltages in figure 4 are not connected to one node, a thermometer code is needed to generate the appropriate signals. Cascaded current comparators (see figure 6) generate this thermometer code. Figure 6 shows one stage of the multi-stage current comparator CC. This comparator can be used to generate the appropriate signals for the gain control and shunt switches in order to achieve very low distortion levels. Total current consumption of the comparator can be rather low because only the actual current stages, for a specific input current, are turned on. Let us assume that the transistors M9 and M12 are in the conducting state. The current comparator consists of two current mirrors. The output currents of the PMOS current mirror (PCM, comprising Ml, M2, M3, M4, M9, M10 and Invl) are equal and controlled by the control means (11, figure 1). The output currents of the ΝMOS current mirror (ΝCM, comprising M5, M6, M7, M8, Mi l, M12 and Inv2) or PMOS mirror are weighted and the ΝMOS mirror is not weighted, or a combination of this, in such a way that the output voltage of stage n varies from Vss to Vdd after stage n-1 and before stage n+1, for increasing values of IAGC. If a current stage does not have to make a decision, that is as long as the output voltage does not have to change, the current through this stage can be reduced to zero in order to save power. So, current flows through the stage only when the stage needs to be active. To achieve this, stage n-2 activates stage n when its output voltage goes up and stage n+2 deactivates stage n when its output voltage goes up. Note that instead of stage n-2 and n+2 also stage n-1 and n+1 could be used to activate and deactivate stage n. For a correct operation of the current stages as described above, transistor M10 and Mil are not needed. However, the output voltage of a current stage drives two MOS transistors in a gain stage (figure 4). In order to enable the gain to vary gradually, the MOS transistors need to be, preferably softly, switched. Therefore the slope of the output voltage of a current stage versus the input current (Iagc) is of interest. For stage n, this slope is mainly determined by the output resistance of the PMOS current-source M3 in parallel with the output resistance of the NMOS current source M7. At certain output voltages, transistor M9 and M12 can operate as cascode transistors resulting in a strongly varying output resistance. Thanks to M10 and Ml 1, the output resistance of the PMOS and NMOS current source are in parallel at any value of the output voltage. As a consequence, the above-mentioned slope remains constant. The output voltage Vout (n) of the multistage current comparator CC is used as one of the tune voltages Vtunel, Vtune2, Vtune3 as shown in the embodiment of Fig. 4. The gain stage 47x exhibits its lowest amplification factor when all the tune voltages Vtunel, Vtune2 and Vtune3 are low: The transistors Ml, M2 and M3 are non-conductive and the transistors M4 and M5 are conductive. The control voltage Vctrl of the shunt switches (SW5x) may also be generated by the multi stage current comparator CC. At the instant Vtune3 became low, Vctrl has to be become high to activate the shunt switches, such that the terminal receiving the input voltage +Vin is connected to the node supplying the output voltage +Vout, and the terminal receiving the input voltage -Vin is connected to the node supplying the output voltage -Vout. It is to be noted that herein above the invention has been described on the basis of some examples. As mentioned before, the amplifier of the invention can be used at different places. For example as an IF amplifier for video or audio signals in a tuner, or as an amplifier of a baseband video or audio signal.

Claims: CLAIMS: 1. Tuner (T) for receiving an input signal (sa) and supplying an output signal (sv), comprising an input amplifier (1), a mixer (3) coupled to the input amplifier (1) for supplying an IF signal, and a multistage amplifier (7) for amplifying the LF signal, the multi stage amplifier (7) comprising a series arrangement of at least two gain stages (271, 272, 273) , and supplying the output signal (sv), and gain control means (11) for controlling the multistage amplifier (7), characterized in that the multistage amplifier (7) further comprises switching means (SW2, SW3, SW3') parallel to the gain stages (271, 272, 273) for shunting at least one of the gain stages (271, 272, 273). 2. Tuner as claimed in claim 1, characterized in that the multistage amplifier (7) comprises at least three gain stages (271, 272, 273) arranged in series. 3. Tuner as claimed in claim 2, characterized in that the switching means (SW2, SW3, SW3') comprise a switch (SW3) coupled in parallel with at least two gain stages (272, 273). 4. Multistage amplifier (7) for use in a tuner as claimed in claim 1. 5. Multistage amplifier (7) as claimed in claim 4, characterized in that the multistage amplifier (7) comprises a cascaded current comparator (CC) for generating tuning signals (Vtunel, Vtune2, Vtune3) for the gain stages (271, 272, 273). 6. Cascaded current comparator (CC) for use in a multistage amplifier (7) as claimed in claim 5, characterized in that the cascaded current comparator (CC) comprise means to turn off inactive gain stages.

Inventors: MENSINK CLEMENS H J, VAN LAMMEREN JOHANNES P M, MENSINK, CLEMENS, H., J., VAN LAMMEREN, JOHANNES, P., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H03G1
IPC codes: H03F3, H03G1, H03G3