# US 9,973,198 B2 · Telescopic amplifier with improved common mode settling > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 9,973,198 B2 - Document ID: US9973198B2 - Status: Granted - Kind: B2 - Application number: 15/495,786 - Filed: 2017-04-24 - Published: 2017-08-10 - Granted: 2018-05-15 - Earliest claimed: 2014-08-27 - Assignees: Texas Instruments Incorporated - Applicants: Texas Instruments Incorporated - Inventors: Roswald Francis - Web page: https://patentgrep.com/patent/US9973198B2 ## CPC - H03M 1/001: {Analogue/digital/analogue conversion} - H03F 3/45183: {Long tailed pairs (H03F3/4521, H03F3/45237 take precedence)} - H03F 3/45188: {Non-folded cascode stages} - H03F 3/45237: {Complementary long tailed pairs having parallel inputs and being supplied in series} - H03F 3/45636: {by using feedback means (H03F3/45744 takes precedence)} - H03F 3/505: {with field-effect devices} - H03M 1/1245: {Details of sampling arrangements or methods} - H03F 3/45: Differential amplifiers (differential sense amplifiers G11C7/062) - H03F 3/45479: {characterised by the way of common mode signal rejection} - H03F 2203/45022: One or more added resistors to the amplifying transistors in the differential amplifier - H03F 2203/45188: the differential amplifier contains one or more current sources in the load - H03F 2203/45404: the CMCL comprising capacitors containing, not in parallel with the resistors, an addition circuit - H03F 2203/45406: the CMCL comprising a common source node of a long tail FET pair as an addition circuit - H03F 2203/45421: the CMCL comprising a switched capacitor addition circuit - H03F 2203/45461: the CSC comprising one or more switched capacitors - H03F 2203/45514: the FBC comprising one or more switched capacitors, and being coupled between the LC and the IC - H03F 2203/45634: the LC comprising one or more switched capacitors - H03M 1/164: {the steps being performed sequentially in series-connected stages (H03M1/161 takes precedence)} - H03M 1/44: Sequential comparisons in series-connected stages with change in value of analogue signal - H03M: CODING; DECODING; CODE CONVERSION IN GENERAL (using fluidic means F15C4/00; optical analogue/digital converters G02F7/00; coding, decoding or code conversion, specially adapted for particular applications, see the relevant subclasses, e.g. G01D, G01R, G06F, G06T, G09G, G10L, G11B, G11C, H04B, H04L, H04M, H04N; ciphering or deciphering for cryptography or other purposes involving the need for secrecy G09C) - H03F: AMPLIFIERS ## Abstract Telescopic amplifier circuits are disclosed. In an embodiment, a telescopic amplifier includes an input stage for receiving differential input signals, an output stage for outputting differential output signals at the drains of a first output transistor and a second output transistor, a tail current transistor coupled to sources of a first input transistor and a second input transistor, a common mode feedback circuit coupled to the differential output signals and outputting a common mode output signal, and a circuit element coupled between the common mode output signal and a gate of the tail current transistor. In an embodiment the circuit element is a resistor. In another embodiment the circuit element is a source follower transistor. In additional embodiments a phase margin of the common mode feedback open loop gain of the amplifier is determined by the value of the resistor. Additional embodiments are disclosed. ## Claims ### Claim 1 A telescopic amplifier, comprising: an input stage comprising a first input transistor having a gate terminal coupled to a positive input terminal, and a second input transistor having a gate terminal coupled to a complementary input terminal; an output stage comprising a first output transistor having a source coupled to a drain of the first input transistor, and a second output transistor having a source coupled to a drain of the second input transistor, a first output terminal coupled to the drain of the first output transistor, and a second output terminal coupled to the drain of the second output transistor; a tail current transistor having a drain coupled to a common source node coupled to the source of each of the first and second input transistors, and having a gate terminal coupled to a tail gate node; a common mode feedback circuit having a first feedback input coupled to the first output terminal, and a second feedback input coupled to the second output terminal, and having a common mode reference signal input; and having a common mode output; and a resistor coupled between the common mode output and the tail gate node. ### Claim 2 (depends on 1) The telescopic amplifier of claim 1, and further comprising a switched capacitor sampling circuit coupled to the first output terminal and to the second output terminal, and having a first sampling transistor and a first positive output capacitor coupled to sample the voltage at the first output terminal, and having a second sampling transistor and a first complementary output capacitor coupled to sample the voltage at the second output terminal, responsive to a clock signal coupled to the gates of the first sampling transistor and the second sampling transistor. ### Claim 3 (depends on 2) The telescopic amplifier of claim 2, wherein the common mode feedback circuit further comprises a switched capacitor circuit. ### Claim 4 (depends on 3) The telescopic amplifier of claim 3 wherein the common mode feedback circuit further comprises: a first transistor coupled between the first output terminal and a first plate of a first sample hold capacitor; a second transistor coupled between a reference common mode input and the first plate of the first sample hold capacitor; a third transistor coupled between an common mode output signal and a bias node that is coupled to a second plate of the first sample and hold capacitor; a fourth transistor coupled between the bias node and a bias voltage input terminal; a fifth transistor coupled between the second output terminal and a first plate of a second sample hold capacitor, the second sample hold capacitor having a second plate coupled to the bias node; a sixth transistor coupled between the reference common mode input and the first plate of the second sample hold capacitor; a first common mode feedback capacitor having a first plate coupled to the first output terminal, and a second plate coupled to the common mode output; a second common mode feedback capacitor having a first plate coupled to the second output terminal and a second plate coupled to the common mode output; and the second, fourth and sixth transistors each having a gate coupled to an inverted clock signal, and the first, third and fifth transistors each having a gate coupled to a clock signal, the clock signal being non-overlapping with the inverted clock signal. ### Claim 5 (depends on 4) The telescopic amplifier of claim 4, whereby when the inverted clock signal is active, the voltage across the first sample hold capacitor is a reference common mode voltage received at the reference common mode input minus a bias voltage received at the bias voltage input, and the voltage across the second sample hold capacitor is the reference common mode voltage received at the reference common mode input minus the bias voltage received at the bias voltage input. ### Claim 6 (depends on 5) The telescopic amplifier of claim 5, whereby when the clock signal is active, the first common mode feedback capacitor is shorted to the first sample hold capacitor, and the second common mode feedback capacitor is shorted to the second sample hold capacitor. ### Claim 7 (depends on 4) The telescopic amplifier of claim 4, wherein a common mode feedback open loop gain transfer function of the telescopic amplifier has a dominant pole due to a load capacitance comprising a sum of the first positive output capacitor and a routing capacitance at the first output terminal, and the common mode feedback open loop gain transfer function of the telescopic amplifier further has a non-dominant pole due to the resistor. ### Claim 8 (depends on 7) The telescopic amplifier of claim 7, wherein the non-dominant pole is located at a frequency P2 that is approximately equal to a quantity (CCM+CTAIL/RpoleCCMCTAIL), where a capacitance CCM is a sum of the first and second common mode feedback capacitors and the first and the second sample and hold capacitors, a capacitance CTAIL is a sum of the gate to source capacitance of the tail transistor plus the gate to drain capacitance of the tail transistor plus a parasitic routing capacitance at the gate of the tail transistor, and a resistance Rpole is a value of the resistor. ### Claim 9 (depends on 8) The telescopic amplifier of claim 8, where a value for Rpole, the resistor, is a value needed to place the non-dominant pole to provide critical damping of the common mode feedback open loop gain of the telescopic amplifier. ### Claim 10 (depends on 8) The telescopic amplifier of claim 8, where a value for Rpole, the resistor, is determined to be approximately equal to a quantity (CL/2β2CTAILGmntail), where a capacitance CTAIL is a sum of the gate to source capacitance of the tail transistor plus the gate to drain capacitance of the tail transistor plus a parasitic routing capacitance at the gate of the tail transistor, a capacitance CL is a sum of the first output capacitor at the first output terminal plus the parasitic routing capacitance at the first output terminal, Gmntail is equal to a transconductance of the tail transistor, and β is a ratio (CCM/CCM+CTAIL) where a capacitance CCM is a sum of the first and second common mode feedback capacitors and the first and the second sample and hold capacitors. ### Claim 11 A telescopic amplifier, comprising: a differential input stage for receiving a positive input signal and a complementary input signal comprising a first input transistor having a gate terminal coupled to a positive input terminal, and a second input transistor having a gate terminal coupled to a complementary input terminal, the first input transistor having a drain and the second input transistor having a drain; a differential output stage for outputting a positive output signal and a complementary output signal comprising a first output transistor having a source coupled to the drain of the first input transistor, and a second output transistor having a source coupled to the drain of the second input transistor and having, and a first output terminal coupled to the drain of the first output transistor, and a second output terminal coupled to the drain of the second output transistor; a tail current transistor having a drain coupled to a common source node coupled to a source of each of the first and the second input transistors, and having a gate terminal coupled to a tail gate node; a common mode feedback circuit having a first feedback input coupled to the first output terminal, and a second feedback input coupled to the second output terminal, and having a common mode reference signal input; and having a common mode output; and a circuit element coupled between the common mode output and the tail gate node, the circuit element being at least one of a source follower transistor and a resistor. ### Claim 12 (depends on 11) The telescopic amplifier of claim 11, wherein the circuit element is the source follower transistor. ### Claim 13 (depends on 11) The telescopic amplifier of claim 11, wherein the circuit element is the resistor. ### Claim 14 (depends on 13) The telescopic amplifier of claim 13, wherein the common mode feedback circuit is a switched capacitor circuit that further comprises: a first transistor coupled between the first output terminal and a first plate of a first sample hold capacitor; a second transistor coupled between a reference common mode input and the first plate of the first sample hold capacitor; a third transistor coupled between an common mode output and a bias node; a fourth transistor coupled between the bias node and a bias voltage input terminal, the bias node further coupled to a second plate of the first sample hold capacitor; a fifth transistor coupled between the second output terminal and a first plate of a second sample hold capacitor, the second sample hold capacitor having a second plate coupled to the bias node; a sixth transistor coupled between the reference common mode input and the first plate of the second sample hold capacitor; a first common mode feedback capacitor having a first plate coupled to the first output terminal, and a second plate coupled to the common mode output; a second common mode feedback capacitor having a first plate coupled to the second output terminal and a second plate coupled to the common mode output; and the second, fourth and sixth transistors each having a gate coupled to an inverted clock signal, and the first, third and fifth transistors having a gate coupled to a clock signal, the clock signal being non-overlapping with the inverted clock signal. ### Claim 15 (depends on 14) The telescopic amplifier of claim 14, and further comprising a switched capacitor sampling circuit coupled to the first output terminal and to the second output terminal, and having a first sampling transistor and a first positive output capacitor coupled to sample the voltage at the first output terminal, and having a second sampling transistor and a first complementary output capacitor coupled to sample the voltage at the second output terminal, responsive to the clock signal. ### Claim 16 (depends on 15) The telescopic amplifier of claim 15, where a value for the resistor is determined so as to place the non-dominant pole to provide critical damping of a common mode feedback open loop gain of the telescopic amplifier. ### Claim 17 (depends on 16) The telescopic amplifier of claim 16, where a value for the resistor is determined to be approximately equal to a quantity CL/2β2CTAILGmntail, where a capacitance CTAIL is a sum of a gate to source capacitance of the tail transistor plus a gate to drain capacitance of the tail transistor plus a parasitic routing capacitance at the gate of the tail transistor, a capacitance CL is a sum of the first positive output capacitor plus the parasitic routing capacitance at the first output terminal, Gmntail is a transconductance of the tail transistor, and β is a ratio CCM/CCM+CTAIL, where a capacitance CCM is a sum of the first and second common mode feedback capacitors, and the first and the second sample and hold capacitors. ### Claim 18 A pipelined ADC converter, comprising: an input terminal for receiving an analog input voltage; an N-bit ADC coupled to the input terminal for outputting a quantized digital signal; an N-bit DAC coupled to the quantized digital signal and outputting an analog voltage corresponding to the quantized digital signal; a sample and hold circuit coupled to sample and hold an analog input voltage received at the input terminal; a summer circuit coupled to the sample and hold circuit and to the analog voltage corresponding to the quantized digital signal, outputting a difference voltage that is a residue voltage obtained from subtracting the analog voltage corresponding to the quantized digital signal from the sample and hold analog voltage; and a telescopic amplifier coupled to the summer circuit to amplify the residue voltage, further comprising: a differential input stage for receiving a positive input signal and a complementary input signal comprising a first input transistor having a gate terminal coupled to a positive input terminal, and a second input transistor having a gate terminal coupled to a complementary input terminal, the first input transistor having a drain and the second input transistor having a drain; a differential output stage for outputting a positive output signal and a complementary output signal comprising a first output transistor having a source coupled to the drain of the first input transistor, and a second output transistor having a source coupled to the drain of the second input transistor, and a first output terminal coupled to the drain of the first output transistor, and a second output terminal coupled to the drain of the second output transistor; a tail current transistor having a drain coupled to a common source node coupled to a source of each of the first and the second input transistors, and having a gate terminal coupled to a tail gate node; a common mode feedback circuit having a first feedback input coupled to the first output terminal, and a second feedback input coupled to the second output terminal, and having a common mode reference signal input; and having a common mode output; and a resistive circuit element coupled between the common mode output and the tail gate node. ### Claim 19 (depends on 18) The pipelined ADC converter of claim 18, wherein the circuit element in the telescopic amplifier further comprises a resistor. ### Claim 20 (depends on 19) The pipelined ADC converter of claim 19, wherein the common mode feedback open loop gain transfer function of the telescopic amplifier has a dominant pole due to a load capacitance comprising the sum of an output capacitor coupled to the first output terminal and a routing capacitance at the first output terminal, and the common mode feedback open loop gain transfer function has a non-dominant pole due to the resistor.