I

C +

V

IN

r

E ) RE

or

R

E +

V

IN

I

C

* rE

OPA615

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

DISCUSSION OF PERFORMANCE

The OPA615, which contains a wideband Operational

OPERATIONAL TRANSCONDUCTANCE

Transconductance

Amplifier

(OTA)

and

a

fast

AMPLIFIER (OTA) SECTION

sampling comparator (SOTA), represents a complete

subsystem for very fast and precise DC restoration,

OVERVIEW

offset clamping and correction to GND or to an

adjustable reference voltage, and low frequency hum

The symbol for the OTA section is similar to that of a

suppression

of

wideband

operational

or

buffer

bipolar transistor, and the self-biased OTA can be

amplifiers.

viewed as either a quasi-ideal transistor or as a

voltage-controlled current source. Application circuits

Although the IC was designed to improve or stabilize

for the OTA look and operate much like transistor

the performance of complex, wideband video signals,

circuits—the

bipolar

transistor

is

also

a

it can also be used as a sample-and-hold amplifier,

voltage-controlled current source. Like a transistor, it

high-speed integrator, peak detector for nanosecond

has three terminals: a high-impedance input (base)

pulses, or as part of a correlated double sampling

optimized for a low input bias current of 0.3

μA, a

system. A wideband Operational Transconductance

low-impedance

input/output

(emitter),

and

the

Amplifier (OTA) with a high-impedance cascode

high-impedance current output (collector).

current source output and a fast and precise

sampling

comparator

sets

a

new

standard

for

The

OTA

consists

of

a

complementary

buffer

high-speed sampling applications.

amplifier and a subsequent complementary current

mirror. The buffer amplifier features a Darlington

Both the OTA and the sampling comparator can be

output stage and the current mirror has a cascoded

used as stand-alone circuits or combined to create

output.

The

addition

of

this

cascode

circuitry

more complex signal processing stages such as

increases the current source output resistance to

sample-and-hold amplifiers. The OPA615 simplifies

1.2M

Ω. This feature improves the OTA linearity and

the

design

of

input

amplifiers

with

high

hum

drive capabilities. Any bipolar input voltage at the

suppression; clamping or DC-restoration stages in

high impedance base has the same polarity and

professional broadcast equipment, high-resolution

signal level at the low impedance buffer or emitter

CAD monitors and information terminals; and signal

output. For the open-loop diagrams, the emitter is

processing stages for the energy and peak value of

connected to GND; the collector current is then

nanoseconds pulses. This device also eases the

determined by the voltage between base and emitter

design of high-speed data acquisition systems behind

times the transconductance. In application circuits

a CCD sensor or in front of an analog-to-digital

(Figure 36b), a resistor RE between the emitter and

converter (ADC).

GND is used to set the OTA transfer characteristics.

An external resistor on the SO-14 package, RQ,

The following formulas describe the most important

allows the user to set the quiescent current. RQ is

relationships. re is the output impedance of the buffer

connected from Pin 1 (IQ adjust) to –VCC. It

amplifier (emitter) or the reciprocal of the OTA

determines the operating currents of the OTA section

transconductance. Above ±5mA, the collector current,

and controls the bandwidth and AC behavior as well

IC, will be slightly less than indicated by the formula.

as the transconductance of the OTA.

Besides the quiescent current setting feature, a

(1)

Proportional-to-Absolute-Temperature (PTAT) supply

current control will increase the quiescent current

The RE resistor may be bypassed by a relatively large

versus

temperature.

This

variation

holds

the

capacitor to maintain high AC gain. The parallel

transconductance (gm) of the OTA and comparator

combination of RE and this large capacitor form a

relatively constant versus temperature. The circuit

high-pass filter, enhancing the high frequency gain.

parameters listed in the specification table are

Other cases may require an RC compensation

measured with RQ set to 300Ω, giving a nominal

network

in

parallel

to

RE

to

optimize

the

quiescent current at 13mA. While not always shown

high-frequency response. The large-signal bandwidth

in the application circuits, this RQ = 300Ω is required

(VO = 1.4VPP) measured at the emitter achieves

to get the 13mA quiescent operating current.

770MHz. The frequency response of the collector is

directly related to the resistor value between the

collector and GND; it decreases with increasing

resistor values, because of the low-pass filter formed

with the OTA C-output capacitance.

12

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