Telescopic amplifier with improved common mode settling

Patent metadata

Field	Value
Publication number	US-9634685-B2
Application number	US-201615258237-A
Country	US
Kind code	B2
Filing date	Sep 7, 2016
Priority date	Aug 28, 2013
Publication date	Apr 25, 2017
Grant date	Apr 25, 2017

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.

First claim

What is claimed is:

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 having a first current source coupled to a drain of the first output transistor, and a second output transistor having a source coupled to a drain of the second input transistor, and having a second current source coupled to a drain of the second output 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, the first and second output transistor each having gates coupled to a bias voltage terminal;
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, having a source coupled to a ground potential, 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, the common mode feedback circuit further having a common mode reference signal input, and a common mode output; and
a resistor coupled between the common mode output and the tail gate node.

2. The telescopic amplifier of claim 1 , further comprising a switched capacitor sampling circuit coupled to the first output terminal and to the second output terminal, the switched capacitor sampling circuit having a first sampling transistor and a first positive output capacitor coupled to sample the voltage at the first output terminal, the switched capacitor sampling circuit further 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 gates of the first sampling transistor and the second sampling transistor.

3. The telescopic amplifier of claim 2 , wherein the common mode feedback circuit further comprises a switched capacitor circuit.

4. 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 and hold capacitor;
a second transistor coupled between a reference common mode input and the first plate of the first sample and hold capacitor;
a third transistor coupled between the common mode output 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 and hold capacitor, the second sample and 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 and 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.

5. The telescopic amplifier of claim 4 , wherein when the inverted clock signal is active, the voltage across the first sample and 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 and 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.

6. The telescopic amplifier of claim 5 , wherein when the clock signal is active, the first common mode feedback capacitor is shorted to the first sample and hold capacitor, and the second common mode feedback capacitor is shorted to the second sample and hold capacitor.

7. 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.

8. The telescopic amplifier of claim 7 , wherein the non-dominant pole is located at a frequency P 2 that is approximately equal to a quantity (C CM +C TAIL /RpoleC CM C TAIL ), where a capacitance C CM is a sum of the first and second common mode feedback capacitors and the first and the second sample and hold capacitors, a capacitance C TAIL 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.

9. 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.

10. The telescopic amplifier of claim 8 , where a value for Rpole, the resistor, is determined to be approximately equal to a quantity (C L /2β 2 C TAIL Gmntail), where a capacitance C TAIL 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 C L 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 (C CM /C CCM +C TAIL ) where a capacitance C CM is a sum of the first and second common mode feedback capacitors and the first and the second sample and hold capacitors.

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 having a first current source coupled to a drain of the first output transistor, and a second output transistor having a source coupled to the drain of the second input transistor and having a second current source coupled to a drain of the second output 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

Assignees

• Texas Instruments Inc

Inventors

• Francis Roswald

Classifications

• H03F2203/45188

  the differential amplifier contains one or more current sources in the load · CPC title

• H03F2203/45406

  the CMCL comprising a common source node of a long tail FET pair as an addition circuit · CPC title

• H03M1/44

  Sequential comparisons in series-connected stages with change in value of analogue signal · CPC title

• H03F2203/45634

  the LC comprising one or more switched capacitors · CPC title

• H03F3/45479

  characterised by the way of common mode signal rejection · CPC title