OTA (operational transconductance amp.) [grise, lecture14, - - PowerPoint PPT Presentation

OTA (operational transconductance amp.)

[grise, lecture14, AN6077.1, LM13700, (AN6668.2)]

 The OTA is a transconductance type device

 the input voltage controls an output current by means

• f the device transconductance gm

 gm is controlled by an external current, the amplifier bias

current, Iabc

 an output voltage can be derived from the output current

by simply driving a resistive load for a BJT OTA: gm=Iabc/2VT which is the transconductance

• f a BJT differential pair

 Basic voltage amp (with feedback; no load here!)...  and if gmR1>>1, gmR2>>1

𝑆𝑗𝑜 = 𝑕𝑛𝑆1 + 1 𝑕𝑛 𝑆𝑗𝑜 ≅ 𝑆1

An all-OTA transistor (note: the first OTA is without feedback!)

• With OTAs it is possible to design active filters which

can be controlled (via the Iabc input) over a number of key parameters

• e.g. (the second OTA is configured as a voltage variable

resistor)

• another example:

1 1 2 2 2

V

• yet another example:

 Internal structure (simplified)

W, X, Y, and Z are current mirrors

 Internal structure (LM13700)

• D1, D4, D5, and

D6 are diode- connected BJT

• D2, D3: see later
• current mirrors

are Wilson

• Q12 and Q13 are

an optional output buffer

(Old) limitation: input voltage swing

• as for any BJT differential pair, the input voltage swing

is limited to about 25 mV (the problem applies if the OTA is being used in open-loop configuration and gm is small) Transfer characteristics (Iout vs Vin):

Transfer characteristics:

 Improvement for the input voltage swing [LM13700]

• recent OTAs use internal linearizing diodes at the input

• suppose the input signal is a current IS (i.e. Rgen is large)
• neglect base currents

Vin=

= DV on the baes Vin = DV on the diodes = DV on the bases =>

Non-idealities (besides those of conventional op-amp)

• change in output offset with IABC
• change in output offset with Idiodes
• change in gain with Idiodes
• ...