Non-linear amplifier

Abstract: The present invention provides a single-ended non-linear amplifier which clamps its output signal to a constant voltage level independent of variations of process and temperature, said amplifier comprising: operational amplifier means having an input node, and an output node for producing an output signal; an input resistor connected between an input terminal and the input node; said input terminal being connected to receive an input signal; a feedback resistor connected between the input node and the output node; a clamping network formed of a P-channel clamping transistor and an N-channel clamping transistor; said P-channel clamping transistor having its conduction path electrodes connected between the input node and the output node and having its gate connected to a P-bias signal; and said N-channel clamping transistor having its conduction path electrodes connected between the input node and the output node and having its gate connected to receive an N-bias signal; characterised by clamp biasing circuit means being responsive to a first reference voltage for generating the P-bias signal and being responsive to a second reference voltage for generating the N-bias signal, said P-bias signal being one threshold drop below said first reference voltage, said N-bias signal being one threshold drop above said second reference voltage.

Description: This invention relates aenerally to a non-linear amplifier which is responsive to large input signal for clipping or clamping its output signal to a constant voltage level independent of variations in process and temperature. In the prior art, there are known non-linear amplifiers of the type in which their outputs are linear for small signal levels and are clipped or clamped to a known voltage level for large signal levels. However, the difficulty encountered with such prior art amplifiers was that the known voltage level at which the clipping or clamping occurred was not constant and varied substantially over process and temperature conditions. It would therefore be desirable to provide a non-linear amplifier which has a clipping/clamping voltage level which is substantially constant over all process and temperature conditions. The non-linear amplifier of the present invention includes four clamping transistors connected in a feedback loop of an operational amplifier. The clamping transistors function as diodes for large input signals so as to clamp the output signal to a constant voltage level. Japanese Patent Abstract No. 57-9114 shows an amplifier having a field effect clamping arrangement between the input and output thereof whereas Japanese Patent Abstract No. 56-67305 shows a similar arrangement with bypolar clamping means. Accordingly, the present invention provides a single-ended non-linear amplifier which clamps its output signal to a constant voltage level independent of variations of process and temperature, said amplifier comprising: operational amplifier means having an input node, and an output node for producing an output signal; an input resistor connected between an input terminal and the input node; said input terminal being connected to receive an input signal; a feedback resistor connected between the input node and the output node; a clamping network formed of a P-channel clamping transistor and an N-channel clamping transistor; said P-channel clamping transistor having its conduction path electrodes connected between the input node and the output node and having its gate connected to a P-bias signal; said N-channel clamping transistor having its conduction path electrodes connected between the input node and the output node and having its gate connected to receive an N-bias signal, and clamp biasing circuit means being responsive to a first reference voltage for generating the P-bias signal and being responsive to a second reference voltage for generating the N-bias signal; characterised in that: said P-bias signal is one threshold drop below said first reference voltage, and said N-bias signal is one threshold drop above said second reference voltage; said clamp biasing circuit means comprising: a bias signal generating P-channel transistor, and a bias signal generating N-channel transistor, wherein when the threshold voltage of the clamping transistors changes, the levels of the N-bias signal and the P-bias signal change proportionally. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings with like reference numerals indicating corresponding parts throughout, wherein: Figure 1 is a schematic circuit diagram of a fully differential non-linear amplifier embodying the present invention; Figure 2 is a schematic circuit diagram of a clamp biasing circuit for use with the amplifier of Figure 1; Figure 3 illustrates the dc characteristics for each single-ended side of the amplifier of Figure 1; Figure 4 are waveforms illustrating the differential output signal in response to a small differential input signal; and Figure 5 are waveforms illustrating the differential output signal in response to a large differential input signal. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in detail to the drawings, there is shown in Figure 1 a schematic circuit diagram of a fully differential non-linear amplifier 10. The non-linear amplifier 10 functions as a linear inverting amplifier to provide a differential output signal having a gain for two for small differential input signals and functions to clamp or clip the differential output signal to a constant voltage level independent of variations in process and temperature for large differential input signals. The non-linear amplifier 1⊘ is comprised of a double-ended operational amplifier 12 which has a pair of input nodes A, B and a pair of output nodes C, D. The input node A is coupled to a first input terminal 14 via a first input resistor R1, and the input node B is coupled to a second input terminal 16 via a second input resistor R2. The input terminals 14 and 16 are connected to receive a differential input signal VIN. Each of the resistors R1 and R2 has a resistance value of R. The output node C is connected to a first output terminal 18, and the output node D is connected to a second output terminal 2⊘. The output terminals 18 and 2⊘ produce a differential output signal VOUT. As is conventional, the single-ended voltage on output terminal 18 is inverted from the single-ended input signal on input terminal 14. Similarly, the single-ended output signal on output terminal 2⊘ is inverted from the single-ended output signal on input terminal 16. Therefore, it should be understood that only one side of the operational amplifier 12 can be used so as to form a single-ended non-linear amplifier. The non-linear amplifier 1⊘ further includes a first feedback resistor R3, a first clamping network CN1, a second feedback resistor R4, and a second clamping network CN2. The first feedback resistor R3 is connected between the first input node A and the first output node C. The first clamping network CN1 is formed of a first P-channel clamping transistor P1⊘ and a first N-channel clamping transistor N1⊘. Similarly, the second feedback resistor R4 is connected between the second input node B and the second output node D. The second clamping network CN2 is formed of a second P-channel clamping transistor P12 and a second N-channel clamping transistor N12. Each of the resistors R3 and R4 has a resistance value of 2R. For convenience, the N-channel transistors will be identified by the letter N followed by a particular reference numeral and the P-channel transistors will be identified by the letter P followed by a particular reference numeral. The conduction paths defined by the source and drain electrodes of the clamping transistors P1⊘ and N1⊘ are also connected between the first input node A and the first output node C in parallel with the first feedback resistor R3. The conduction paths defined by the source and drain electrodes of the clamping transistors P12 and N12 are also connected between the second input node B and the second output node D in parallel with the second feedback resistor R4. The gates of the first and second P-channel clamping transistors P1⊘ and P12 are connected to receive a P-channel bias signal P-BIAS. The gates of the first and second N-channel clamping transistors N1⊘ and N12 are connected to receive an N-channel bias signal N-BIAS. In common implementation, the bulk of the clamping transistors P1⊘ and P12 are connected to the substrate and to a supply voltage or potential VCC, which is typically at +5.⊘ volts, and the bulk of the clamping transistors N1⊘ and N12 are connected to a ground potential VSS. In Figure 2, there is shown a schematic circuit diagram of a clamp biasing circuit 22 for generating the P-channel bias signal P-BIAS and the N-channel bias signal N-BIAS which are used by the first and second clamping networks CN1 and CN2 of the non-linear amplifier 1⊘. The biasing circuit 22 is formed of a P-bias circuit section 24 and an N-channel bias circuit section 26. The P-channel bias circuit section 24 includes a reference current source IBIAS, an N-channel current mirror arrangement formed of N-channel transistors N1-N4, and a gm-connected P-channel MOS transistor P1. The transistor N2 has its gate and drain electrodes connected together and to the source of the transistor N1. The transistor N2 has its source connected to the ground potential VSS. The transistor N1 has its gate and drain electrodes connected together and to one end of the current source IBIAS. The other end of the current source is connected to the supply potential VCC. The transistor N3 has its drain connected to the source of the transistor N4, its gate connected to the gate of the transistor N2, and its source connected to the ground potential VSS. The transistor N4 has its drain connected to the drain of the transistor P1 and its gate connected to the gate of the transistor N1. The transistor P1 has its source connected to a reference voltage P-REFERENCE. The gate and drain electrodes of the transistor P1 are connected together and generate the P-channel bias signal at node E for the gates of the P-channel clamping transistors P1⊘ and P12. Thus, the signal P-BIAS is designed to be one threshold drop V TH below the reference voltage P-REFERENCE. The N-BIAS circuit section 26 includes an N-channel current mirror arrangement formed of N-channel transistors N1, N2, N5 and N6; a P-channel current mirror arrangement formed of P-channel transistors P2-P5; and a gm-connected N-channel MOS transistor N7. The transistor N5 has its drain connected to the source of the transistor N6, its gate connected to the gate of the transistor N2, and its source connected to the ground potential VSS. The transistor N6 has its gate connected to the gate of the transistor N1 and its drain connected to the drain of the transistor P2. The transistor P2 has its source connected to the drain of the transistor P3 and its gate and drain electrodes connected together. The transistor P3 has its source connected to the supply potential VCC and its gate and drain electrodes connected together. The transistor P4 has its source connected also to the supply potential VCC, its gate connected to the gate of the transistor P3, and its drain connected to the source of the transistor P5. The transistor P5 has its gate connected to the gate of the transistor P2 and its drain connected to the drain of the transistor N7. The transistor N7 has its gate and drain electrodes connected together and generates the N-channel bias signal at node F for the gates of the N-channel clamping transistors N1⊘ and N12. The transistor N7 has its source connected to a reference voltage N-REFERENCE. Thus, the signal N-BIAS is designed to be one threshold voltage drop V TH above the reference voltage N-REFERENCE. It should be noted that each of the N-channel transistors N1 through N6 has its source connected to its bulk. Further, the gm-connected N-channel transistor N7 has its bulk connected to the ground potential VSS. Each of the P-channel transistors P2 through P5 has its bulk connected to the supply potential VCC. The bulk of the gm-connected P-channel transistor P1 is also connected to the supply potential VCC. In the embodiment illustrated, the reference voltage P-REFERENCE is selected to be a constant 3.4 volts and the reference voltage N-REFERENCE is selected to be a constant 1.4 volts. Since the bulk of the transistor P1 is tied to the supply potential VCC, the P-channel bias signal appearing at the node E will be approximately 1.⊘ volts below the reference voltage P-REFERENCE or 2.4 volts. Since the substrate of the transistor N7 is tied to the ground potential VSS, the N-channel bias signal appearing at the node F will be approximately 1.⊘ volts above the reference voltage N-REFERENCE or 2.4 volts. Referring to Figure 3, there is shown the dc characteristics for each single-ended side of the non-linear amplifier 1⊘ of Figure 1. The operation of the amplifier 1⊘ will now be explained with reference to Figure 3. Initially, it is assumed that each input node of the nodes A, B defining a null port is at a constant voltage of 2.4 volts. Further, when the differential output voltage VOUT is at zero volts, each of the output nodes C and D will be at an analog reference voltage of 2.4 volts. Thus, the source and drain voltages across all of the clamping transistors P1⊘, P12, N1⊘ and N12 will be at the same potential. Since the signal N-BIAS will be at approximately 2.4 volts, the gate-to-source voltage for the N-channel clamping transistors N1⊘ and N12 will be below the threshold voltage, thereby rendering them to be non-conductive. Similarly, since the signal P-BIAS will also be at approximately 2.4 volts, the gate-to-source voltage for the P-channel clamping transistors P1⊘ and P12 will be above the threshold voltage, thereby rendering them to be likewise non-conductive. By applying a positive ramping input voltage V IN , as illustrated in curve 3⊘ of Figure 3, to the input terminal 14, the output, voltage V OUT on the output terminal 18 will follow the curve 32. As the output signal V OUT decreases below 2.4 volts, the electrode of the transistor N1⊘ connected to the output node C of the amplifier 1⊘ defines its source and the electrode of the transistor P1⊘ connected to the input node A defines its source. When the output signal V OUT decreases below the reference voltage N-REFERENCE or 1.4 volts, the transistor N1⊘ is turned on and functions as a diode so as to clamp the output node C at one volt below the analog reference of 2.4 volts. Further, the transistor P1⊘ will be maintained to be non-conductive during this time since its gate-to-source voltage will always be higher than its threshold voltage. By applying a negative ramping input signal V IN , as illustrated in curve 34 of Figure 3, to the input terminal 16, the output signal V OUT on the output terminal 2⊘ will follow the curve 36. As the output signal V OUT increases above 2.4 volts, the electrode of the transistor P12 connected to the output node D of the amplifier 1⊘ defines its source and the electrode of the transistor N12 connected to the input node B defines its source. When the output signal V OUT increases above the reference voltage P-REFERENCE or 3.4 volts, the transistor P12 is turned on and functions as a diode so as to clamp the output node D at one volt above the analog reference voltage of 2.4 volts. Further, the transistor N12 will be maintained to be non-conductive during this time since its gate-to-source voltage will always be lower than its threshold voltage. It should be apparent that the N-channel clamping transistors N1⊘ and N12 and the P-channel clamping transistors P1⊘ and P12 will operate in a similar manner when the negative and positive ramping input signals (curves 34 and 3⊘) are applied to the input terminals 14 and 16. Thus, the reverse operation of the clamping transistors N1⊘, N12, P1⊘ and P12 will not be described in detail. The clamping point of the present amplifier 1⊘ can be easily changed by adjusting the level of the reference voltages P-REFERENCE and N-REFERENCE. Further, the clamping point of the amplifier will remain substantially constant at the same voltage level independent of variations in process and temperature conditions. This is because when the process and temperature changes the threshold voltage of the clamping transistors N1⊘, P1⊘, N12 and P12, the levels of the bias signal P-BIAS and N-BIAS will be change proportionally. This is achieved by matching geometrically the transistor P1 of the clamp biasing circuit 22 with the clamping transistors P1⊘ and P12. Similarly, the transistor N7 of the clamp biasing circuit 22 is matched geometrically with the N-channel clamping transistors N1⊘ and N12. For example, with particular reference to the N-channel clamping transistor N1⊘, it will be recalled that the N-channel bias signal N-BIAS is made to be one threshold drop V TH above the reference voltage N-REFERENCE. As the process or temperature varies the threshold drop V TH of the clamping transistor N1⊘, the threshold drop V TH in the gm-connected transistor N7 will vary proportionally and thus the clamp voltage of the amplifier will remain substantially at the same level. If the threshold voltage of the clamping transistor N1⊘, for instance, is increased by 1⊘⊘ mV, the level of the N-channel bias signal N-BIAS will also be increased by 1⊘⊘ mV. In Figure 4, there is depicted a waveform of the differential output voltage VOUT (curve 38) of the non-linear amplifier 1⊘ in response to a small differential input signal VIN (curve 4⊘). In this condition, the amplifier is operated in the linear region with a gain of two. In Figure 5, there is illustrated the waveform of the differential output signal VOUT (curve 42) of the amplifier 1⊘ in response to a large differential input signal VIN (curve 44). In this condition, the amplifier 1⊘ is operated in the non-linear region and the differential output voltage is limited to 2.⊘ volts. From the foregoing detailed description, it can thus be seen that the described embodiment provides an improved fully differential non-linear amplifier which is responsive to large differential input signals for clamping its output signal to a constant voltage level independent of variations in process and temperature. The non-linear amplifier is formed of a double-ended operational amplifier, a first input resistor, a second input resistor, a first feedback resistor, a second feedback resistors a first clamping network, and a second clamping network. Each of the first and second clamping networks are formed of a P-channel transistor and an N-channel transistor.

Claims: A single-ended non-linear amplifier which clamps its output signal to a constant voltage level independent of variations of process and temperature, said amplifier comprising: operational amplifier means (12) having an input node, and an output node for producing an output signal; an input resistor (R1) connected between an input terminal and the input node; said input terminal being connected to receive an input signal; a feedback resistor (R3) connected between the input node and the output node; a clamping network (CN1) formed of a P-channel clamping transistor (P10) and an N-channel clamping transistor (N10); said P-channel clamping transistor (P10) having its conduction path electrodes connected between the input node and the output node and having its gate connected to a P-bias signal; said N-channel clamping transistor (N10) having its conduction path electrodes connected between the input node and the output node and having its gate connected to receive an N-bias signal, and clamp biasing circuit means (22) being responsive to a first reference voltage (P-REFERENCE) for generating the P-bias signal and being responsive to a second reference voltage (N-REFERENCE) for generating the N-bias signal; characterised in that: said P-bias signal is one threshold drop below said first reference voltage, and said N-bias signal is one threshold drop above said second reference voltage; said clamp biasing circuit means (22) comprising: a bias signal generating P-channel transistor (P1) and a bias signal generating N-channel transistor (N7), wherein when the threshold voltage of the clamping transistors (P10,N10) changes, the levels of the N-bias signal and the P-bias signal change proportionally. A non-linear amplifier as claimed in Claim 1, wherein said clamp biasing circuit (22) comprises a P-bias circuit section (24) formed of a reference current source (IBIAS), a first current mirror arrangement (N1-N4), and said bias signal generating P-channel transistor (P1), and an N-bias circuit section (26) formed of a second current mirror arrangement (N1, N2, N5, N6), a third current mirror arrangement (P2-P5), and said bias signal generating N-channel transistor (N7). A non-linear amplifier as claimed in Claim 2, wherein said bias signal generating P-channel transistor (P1) is geometrically matched to said P-channel clamping transistor (P10). A non-linear amplifier as claimed in Claim 2 or Claim 3, wherein said bias signal generating N-channel transistor (N7) is geometrically matched to said N-channel clamping transistor (N10). A non-linear amplifier as claimed in any preceding claim, wherein said first P-channel clamping transistor (P10) is turned on only when the output signal on the output node increases above said first reference voltage (P-REFERENCE). A non-linear amplifier as claimed in Claim 5, wherein said first N-channel clamping transistor (N10) is turned on only when the output signal on the output node decreases below said second reference voltage (N-REFERENCE).

Inventors: BREHMER GEOFFREY E, JACKSON HARRY S, BREHMER, GEOFFREY E., JACKSON, HARRY S.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1989
Main classification: H03G11
IPC codes: H03G11, G06G7