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BASIC APPLICATION CIRCUITS

+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

3

2

C

12

Transconductance varies over temperature.

Transconductance remains constant over temperature.

V

OPA615

SBOS299B – FEBRUARY 2004 – REVISED JULY 2005

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 cur-

low impedance loads. The emitter output is not

rent flows out of the C terminal for a positive B-to-E

current-limited or protected. Momentary shorts to

input voltage and into it for negative voltages; 2) A

GND should be avoided, but are unlikely to cause

common emitter amplifier operates in non-inverting

permanent damage.

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 tem-

perature and keeps the AC performance constant; 6)

The OTA is self-biased and bipolar; and 7) The

output current is approximately zero for zero differen-

tial input voltages. AC inputs centered on zero

produce an output current centered on zero.

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; com-

mon-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 equiv-

alent 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

13