+V

CC

(13)

+V

CC

(5)

C

(12)

B

(3)

E

(2)

+1

R

B

R

L

R

B

R

E

V

−

Single Transistor

V+

V

I

V

O

(a) Common Emitter Amplifier

V

O

100

Ω

OTA

V

I

B

E

R

L

R

E

NoninvertingGain

(b) Common−E Amplifier for OTA

Inverting Gain

V

several volts

OS ≈

3

2

C

12

Transconductance varies over temperature.

Transconductance remains constant over temperature.

V

OS ≈ 0

OPA615

www.ti.com...................................................................................................................................... SBOS299E – FEBRUARY 2004 – REVISED SEPTEMBER 2009

Figure 35 shows a simplified block diagram of the

While the OTA function and labeling appear similar to

OPA615 OTA. Both the emitter and the collector

those of a transistor, it offers essential distinctive

outputs offer a drive capability of ±20mA for driving

differences and improvements: 1) The collector

low impedance loads. The emitter output is not

current flows out of the C terminal for a positive

current-limited or protected. Momentary shorts to

B-to-E input voltage and into it for negative voltages;

GND should be avoided, but are unlikely to cause

2)

A

common

emitter

amplifier

operates

in

permanent damage.

non-inverting mode while the common base operates

in inverting mode; 3) The OTA is far more linear than

a bipolar transistor; 4) The transconductance can be

adjusted with an external resistor; 5) As a result of

the PTAT biasing characteristic, the quiescent current

increases as shown in the typical performance curve

vs temperature and keeps the AC performance

constant; 6) The OTA is self-biased and bipolar; and

7) The output current is approximately zero for zero

differential input voltages. AC inputs centered on zero

produce an output current centered on zero.

BASIC APPLICATION CIRCUITS

Most application circuits for the OTA section consist

of a few basic types which are best understood by

analogy to discrete transistor circuits. Just as the

transistor has three basic operating modes—common

emitter, common base, and common collector—the

OTA

has

three

equivalent

operating

modes;

common-E,

common-B,

and

common-C

(see

Figure 36, Figure 37 and Figure 38). Figure 36 shows

the OTA connected as a Common-E amplifier, which

is equivalent to a common emitter transistor amplifier.

Input and output can be ground-referenced without

any biasing. The amplifier is noninverting because a

Figure 35. Simplified OTA Block Diagram

current flowing out of the emitter will also flow out of

the collector as a result of the current mirror shown in

Figure 35.

Figure 36. a) Common Emitter Amplifier Using a Discrete Transistor; b) Common-E Amplifier Using the

OTA Portion of the OPA615

Copyright © 2004–2009, Texas Instruments Incorporated

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