廬
APPLICATION BULLETIN
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AUTOMATIC GAIN CONTROL (AGC)
USING THE DIAMOND TRANSISTOR OPA660
By Christian Henn, Burr-Brown International GmbH
Multiplication of analog signals has long been one of the
most important nonlinear functions of analog circuit tech-
nology. Many signal sources, however, such as CCD sen-
sors, pin diodes, or antennas, deliver weak, oscillating, and
simultaneously wide-band signals. But now a new multipli-
cation method is available. Used as a wide-bandwidth Auto-
matic Gain Control (AGC) application circuit, the integrated
circuit OPA660 varies its own gain to change the signal
amplitude and keep the output signal constant over a wide
input voltage range. The OPA660 thus makes it possible to
control and amplify signals with no additional multiplier.
Important parameters include the differential gain (DG), the
thermally induced pulse distortion, and the signal-to-noise
ratio (S/N).
An analog multiplier delivers an output signal (voltage or
current) that is proportional to the product of two or more
inputs. The application circuit presented here is concerned
primarily with two inputs. In the simplest case, each of the
two inputs can function with both polarities. In this case, the
input voltage swing covers all four quadrants; that is, there
are four polarity combinations. In contrast to a four quadrant
multiplier, a two quadrant multiplier allows only one input
to be connected to a signal of any polarity. The second input
can only process unipolar signals.
Multipliers are nonlinear and thus can not be implemented
as simply and exactly as linear components. In developing
the circuit, various design methods were used depending
upon the accuracy, bandwidth, and justifiable complexity.
Multipliers do have several disadvantages, including linear-
ity errors, temperature dependence, less than ideal crosstalk,
and limited bandwidth, but the multiplication function pre-
sented here functions directly and has variable
transconductance, enabling it to achieve the largest possible
bandwidth.
AGC WITH THE DIAMOND TRANSISTOR
The voltage-controlled current source of the OPA660 from
Burr-Brown has acquired various nicknames according to its
applications:
Operational Transconductance Amplifier (OTA)
Current Conveyor
Diamond Transistor
Ideal Transistor
Macrotransistor
Applications for the OPA660 are usually amplifier
circuits. But although the OPA660鈥檚 connection pin,
I
Q
, adjusts functions primarily as a power supply switch or
漏
quiescent current programmer, it can also be used for mul-
tiplicative applications.
Figure 1 illustrates the dependance of the transconductance
(gm = d(I
OUT
)/d(V
IN
)) upon the resistance, R
QC
. The follow-
ing equation can be derived from the idealized OPA660
model circuits shown in Figure 2.
V
I
QC
=
T
l
n (n)
R
QC
When the temperature voltage (V
T
) is 25.86mV, the quies-
cent current resistance (R
QC
) is 250鈩? and the scale factor (n)
of the transistor R
122
is 10, the cross current I
QC
can be
calculated as follows:
I
QC
=
25.86mV
ln(10) = 238碌A
250鈩?/div>
The quiescent current of the subsequent transistor stages can
be calculated with a scale factor (a) of 7.3 for transistors 31,
32, 81, and 82 to
I
QC
' = a 鈥?I
QC
= 7.3 鈥?238碌A = 1.74mA
I
OUT
(mA)
1.5
R
QC
250
1.0
500
0.5
鈥?0
鈥?5
鈥?0
鈥?.5
10
15
20
V
IN
(mV)
鈥?.0
鈥?.5
V
OUT
= V
IN
M
R
QC
V
OUT
V
IN
M
R
QC
FIGURE 1. Schematic Diagram of the Multiplication
Function.
AB-185
Printed in U.S.A. October, 1993
1993 Burr-Brown Corporation
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