Pentek 6210 Operating Manual page 146

AD603
output signal. The automatic gain control voltage, V
time-integral of this error current. In order for V
the gain) to remain insensitive to short-term amplitude fluctuations
in the output signal, the rectified current in Q1 must, on average,
exactly balance the current in Q2. If the output of A2 is too small
to do this, V will increase, causing the gain to increase, until
AGC
Q1 conducts sufficiently.
Consider the case where R8 is zero and the output voltage V
is a square wave at, say, 455 kHz, which is well above the corner
frequency of the control loop.
During the time V is negative with respect to the base voltage
OUT
of Q1, Q1 conducts; when V
the average collector current of Q1 is forced to be 300 µA, and
the square wave has a duty-cycle of 1:1, Q1's collector current
when conducting must be 600 µA. With R8 omitted, the peak
amplitude of V is forced to be just the V
OUT
typically about 700 mV, or 2 V
hence the amplitude at which the output stabilizes, has a strong
negative temperature coefficient (TC), typically –1.7 mV/°C.
Although this may not be troublesome in some applications, the
correct value of R8 will render the output stable with temperature.
To understand this, first note that the current in Q2 is made
to be proportional to absolute temperature (PTAT). For the
moment, continue to assume that the signal is a square wave.
When Q1 is conducting, V
OUT
voltage that is PTAT and which can be chosen to have an equal
but opposite TC to that of the V
than an application of the "bandgap voltage reference" principle.
When R8 is chosen such that the sum of the voltage across it
and the V of Q1 is close to the bandgap voltage of about 1.2 V,
BE
V will be stable over a wide range of temperatures, provided,
OUT
of course, that Q1 and Q2 share the same thermal environment.
Since the average emitter current is 600 µA during each half-
cycle of the square wave a resistor of 833 Ω would add a PTAT
voltage of 500 mV at 300 K, increasing by 1.66 mV/°C. In prac-
tice, the optimum value will depend on the type of transistor
used and, to a lesser extent, on the waveform for which the
temperature stability is to be optimized; for the inexpensive
2N3904/2N3906 pair and sine wave signals, the recommended
value is 806 Ω.
, is the
AGC
(and thus
AGC
is positive, it is cut off. Since
OUT
of Q1 at 600 µA,
BE
peak-to-peak. This voltage,
BE
is now the sum of V and a
BE
. This is actually nothing more
BE
This resistor also serves to lower the peak current in Q1 when
more typical signals (usually, sinusoidal) are involved, and the
1.8 kHz LP filter it forms with C
due to ripple in V
AGC
sine wave conditions will be higher than for a square wave, since
the average value of the current for an ideal rectifier would be
0.637 times as large, causing the output amplitude to be
1.88 (=1.2/0.637) V, or 1.33 V rms. In practice, the somewhat
OUT
nonideal rectifier results in the sine wave output being regulated
to about 1.4 V rms, or 3.6 V p-p.
The bandwidth of the circuit exceeds 40 MHz. At 10.7 MHz,
the AGC threshold is 100 µV (–67 dBm) and its maximum gain
is 83 dB (20 log 1.4 V/100 µV). The circuit holds its output at
1.4 V rms for inputs as low as –67 dBm to +15 dBm (82 dB),
where the input signal exceeds the AD603's maximum input
rating. For a 30 dBm input at 10.7 MHz, the second harmonic
is 34 dB down from the fundamental and the third harmonic is
35 dB down.
CAUTION
Careful component selection, circuit layout, power-supply
decoupling, and shielding are needed to minimize the AD603's
susceptibility to interference from radio and TV stations, etc. In
bench evaluation, we recommend placing all of the components
in a shielded box and using feedthrough decoupling networks
for the supply voltage. Circuit layout and construction are also
critical, since stray capacitances and lead inductances can form
resonant circuits and are a potential source of circuit peaking,
oscillation, or both.
–10–
helps to minimize distortion
AV
. Note that the output amplitude under
REV. D