AD603
During the time V
OUT
is negative with respect to the base
voltage of Q1, Q1 conducts; when V
OUT
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 V
OUT
is forced to be just the V
BE
of Q1 at
600 µA, typically about 700 mV, or 2 V
BE
peak-to-peak. This
voltage, 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, 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
is now the sum of V
BE
and a
voltage that is PTAT and that can be chosen to have an equal
but opposite TC to that of the V
BE
. This is actually nothing more
than an application of the band gap voltage reference principle.
When R8 is chosen such that the sum of the voltage across it
and the V
BE
of Q1 is close to the band gap voltage of about 1.2 V,
V
OUT
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
practice, 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.
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
AV
helps to minimize distortion
due to ripple in V
AGC
. 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.
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 signals such as those from
radio and TV stations. In bench evaluation, it is recommended
to place all of the components into 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.
Rev. G | Page 19 of 20
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