Sam PalermoAnalog & Mixed-Signal Center

Texas A&M University

ECEN326: Electronic CircuitsFall 2017

Lecture 6: Operational Transconductance Amplifiers (OTAs)

Announcements

• HW4 due today• HW5 due 11/1

2

OpAmps and OTAs

• High voltage gain• High input impedance• Voltage source output

(low impedance)3

OpAmp OTA

• High “voltage” gain• High input impedance• Current source output

(high impedance)

Simple OTA

• Important Parameters• Transconductance• Output Resistance• Differential Gain• Common-Mode Input Range

4

Simple OTA Transconductance: Rigorous Analysis

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Simple OTA Transconductance: Informal Virtual Ground Approach

6

• As the differential circuit is not purely symmetric, we cannot formally assume a virtual ground at node P

• However, if the differential pair transistors M1 and M2 have high output resistance, a virtual ground can be approximated at node P to simplify the analysis

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Simple OTA Output Resistance

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Simple OTA Differential Gain

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Simple OTA Common-Mode Input Range

9

• Common-mode input range set by transistor saturation conditions• Low-end set by tail current source

saturation

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• High-end set by differential pair saturation

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Bipolar Simple OTA

10

• Following a similar procedure

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• High-end Vicm set by keeping differential pair in active mode

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• Low-end Vicm set by keeping tail current source in active mode

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3 Current Mirror OTA

11

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• While Gm has increased by the current mirror factor B, the voltage gain remains the same due to the output resistance being reduced by B-1

• Common-mode input range expression remains the same as the previous simple OTA

Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)

12

• In order to improve the distortion performance, emitter resistors have been added in the input differential pair

• In order to improve the output resistance, emitter resistance has been added to all the current mirror/source transistors

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Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)

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Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)

14

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• Low-end Vicm set by keeping tail current source in active mode

• High-end Vicm set by keeping differential pair in active mode

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• Maximum differential input amp. for good distortion

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Simulating the 3 Current Mirror OTA

15

• To simulate differential amplifiers, use voltage-controlled voltage sources (VCVS or “E” elements) to generate the differential input signal and monitor the current through the load resistor

Single-ended input source

2 VCVS to generate differential inputs

DC voltage source to set input DC level

Monitor I(RL)Note: This example is designed for 2X the Lab 7 Gm

Simulating Transconductance

16

Single-ended input sourceDC=0AC=1

Gain=0.5DC voltage source to set input DC level

• Set Inputs E1 Gain=0.5 and E2 Gain=-0.5• With input source AC=1, simply plot the load resistor current I(RL)

to get the transconductance

Gain=-0.5

Monitor I(RL)

Simulating Transconductance

17

• Gm = 2.01mA/V

Simulating Differential Rind

18

• The differential input resistance is equivalent to the differential input (Vi) divided by the input current, where I use the base current of Q1 or IB(Q1)

• Rind = 159k

Simulating THD

19

• Set input source to differential input amplitude spec • For Lab 7, that is 2V

• Check the THD at 3 different common-mode points• 0V, VCM,min, VCM,max

VCM=0V

VCM=-2V

VCM=2V

Simulating Output Resistance

20

Ground input source

• Ground input source and apply an AC-coupled voltage stimulus at output• With output source AC=1, plot the ratio of V(Vout) over the current

through the coupling capacitorOutput stimulus

Simulating Ro

21

• The output resistance is equivalent to the output stimulus V(Vout) divided by the output current, which is equal to the current through the output capacitor I(C5)

• Ro = 25.4k