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AD603ACHIPS
(Rev.:RevC)

Low Noise, 90 MHz Variable-Gain Amplifier

Analog Devices 

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.

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.

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.

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.

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.

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–

REV. C

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