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

of 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

of a BJT differential pair

Basic voltage amp (with feedback; no load here!)...

and if gmR1>>1, gmR2>>1

𝑅𝑖𝑛 =𝑔𝑚𝑅1+ 1

𝑔𝑚

𝑅𝑖𝑛 ≅ 𝑅1

An all-OTA amplifier (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 basesVin = 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

• ...