AD603
output signal. The automatic gain control voltage, VAGC, is the
time-integral of this error current. In order for VAGC (and thus
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, VAGC will increase, causing the gain to increase, until
Q1 conducts sufficiently.
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 CAV helps to minimize distortion
due to ripple in VAGC. Note that the output amplitude under
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
nonideal rectifier results in the sine wave output being regulated
to about 1.4 V rms, or 3.6 V p-p.
Consider the case where R8 is zero and the output voltage VOUT
is a square wave at, say, 455 kHz, which is well above the corner
frequency of the control loop.
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.
During the time VOUT is negative with respect to the base voltage
of Q1, Q1 conducts; when VOUT is positive, it is cut off. Since
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 VOUT is forced to be just the VBE of Q1 at 600 µA,
typically about 700 mV, or 2 VBE peak-to-peak. This voltage,
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.
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.
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, VOUT is now the sum of VBE and a
voltage that is PTAT and which can be chosen to have an equal
but opposite TC to that of the VBE. This is actually nothing more
than an application of the “bandgap voltage reference” principle.
When R8 is chosen such that the sum of the voltage across it
and the VBE of Q1 is close to the bandgap voltage of about 1.2 V,
VOUT will be stable over a wide range of temperatures, provided,
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
2N3904/2N306 pair and sine wave signals, the recommended
value is 806 Ω.
–10–
REV. C
ADI [ ADI ]
