OPA615 datasheet(12/26 Pages) TI

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DISCUSSION OF PERFORMANCE

OPERATIONAL TRANSCONDUCTANCE

SECTION and OVERVIEW

C +

E ) RE

E +

* rE

OPA615

SBOS299B – FEBRUARY 2004 – REVISED JULY 2005

The OPA615, which contains a wideband Operational

Transconductance Amplifier (OTA) and a fast sam-

AMPLIFIER (OTA) SECTION AND OVERVIEW

pling comparator (SOTA), represents a complete

subsystem for very fast and precise DC restoration,

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 ampli-

bipolar transistor, and the self-biased OTA can be

fiers.

viewed as either a quasi-ideal transistor or as a

Although the IC was designed to improve or stabilize

voltage-controlled current source. Application circuits

the performance of complex, wideband video signals,

for the OTA look and operate much like transistor

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

circuits—the

bipolar

transistor

also

volt-

high-speed integrator, peak detector for nanosecond

age-controlled current source. Like a transistor, it has

pulses, or as part of a correlated double sampling

three

terminals:

high-impedance

input

(base)

system. A wideband Operational Transconductance

optimized for a low input bias current of 0.3

µA, a

Amplifier (OTA) with a high-impedance cascode cur-

low-impedance

input/output

(emitter),

and

the

rent source output and a fast and precise sampling

high-impedance current output (collector).

comparator sets a new standard for high-speed

The OTA consists of a complementary buffer ampli-

sampling applications.

fier and a subsequent complementary current mirror.

Both the OTA and the sampling comparator can be

The buffer amplifier features a Darlington output

used as stand-alone circuits or combined to create

stage and the current mirror has a cascoded output.

more complex signal processing stages such as

The addition of this cascode circuitry increases the

sample-and-hold amplifiers. The OPA615 simplifies

current source output resistance to 1.2M

Ω. This

the design of input amplifiers with high hum sup-

feature improves the OTA linearity and drive capabili-

pression; clamping or DC-restoration stages in pro-

ties. Any bipolar input voltage at the high impedance

fessional broadcast equipment, high-resolution CAD

base has the same polarity and signal level at the low

monitors and information terminals; and signal pro-

impedance buffer or emitter output. For the open-loop

cessing stages for the energy and peak value of

diagrams, the emitter is connected to GND; the

nanoseconds pulses. This device also eases the

collector current is then determined by the voltage

design of high-speed data acquisition systems behind

between

base

and

emitter

times

the

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

transconductance. In application circuits (Figure 36b),

converter.

a resistor RE between the emitter and GND is used to

set the OTA transfer characteristics.

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

allows the user to set the quiescent current. RQ is

The following formulas describe the most important

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

relationships. re is the output impedance of the buffer

mines the operating currents of the OTA section and

amplifier (emitter) or the reciprocal of the OTA

controls the bandwidth and AC behavior as well as

transconductance. Above

±5mA, the collector current,

the transconductance of the OTA.

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

Besides the quiescent current setting feature, a

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 net-

measured with RQ set to 300Ω, giving a nominal

work in parallel to RE to optimize the high-frequency

quiescent current at 13mA. While not always shown

response. The large signal bandwidth (VO = 1.4VPP)

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

measured at the emitter achieves 770MHz. The

to get the 13mA quiescent operating current.

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.

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