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.
Because 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 renders 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
is stable over a wide range of temperatures, provided, of
course, that Q1 and Q2 share the same thermal environment.
Because the average emitter current is 600 μA during each half
cycle of the square wave, a resistor of 833 Ω adds a PTAT
voltage of 500 mV at 300 K, increasing by 1.66 mV/°C. In
practice, the optimum value depends 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 is higher than for a square wave because the
average value of the current for an ideal rectifier is 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 maximum input rating of the
AD603. 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 from the fundamental.
CAUTION
Careful component selection, circuit layout, power supply
decoupling, and shielding are needed to minimize the susceptibility
of the AD603 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 use
feedthrough decoupling networks for the supply voltage. Circuit
layout and construction are also critical because stray capacitances
and lead inductances can form resonant circuits and are a
potential source of circuit peaking, oscillation, or both.
Rev. H | Page 18 of 24
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