
AD600/AD602
This is a Class AB amplifier. As V
IN
 increases in a positive 
direction, Q1 conducts more heavily and its r
e
 becomes lower 
while Q2 increases. Conversely, increasingly negative values of 
V
IN
 result in the r
e
 of Q2 decreasing, while the r
e
 of Q1 increases. 
The design is chosen such that the net emitter resistance is 
essentially independent of the instantaneous value of V
IN
, 
resulting in moderately low distortion. Low values of resistance 
and moderately high bias currents are important in achieving 
the low noise, wide bandwidth, and low distortion of this 
preamplifier. Heavy decoupling prevents noise on the power 
supply lines from being conveyed to the input of the X-AMP.  
Table 4. Measured Preamplifier Performance 
Measurement  
Gain (f = 30 MHz)  
Bandwidth (3 dB)  
Input Signal for 1 dB Compression 
Distortion  
V
IN
 = 200 mV p-p  
V
IN
 = 500 mV p-p  
System Input Noise  
Spectral Density (NSD)  
(Preamp plus X-AMP)  
Input Resistance  
Input Capacitance  
Input Bias Current  
Power Supply Voltage  
Quiescent Current  
Rev. E | Page 17 of 28 
HD2  
HD3  
HD2  
HD3  
Value  
6  
250  
1 
0.27  
0.14  
0.44  
0.58  
1.03  
1.4  
15  
±150  
±5  
15  
Unit  
dB  
MHz  
V p-p 
%  
%  
%  
%  
nV/√Hz  
kΩ  
pF  
μA  
V  
mA  
A LOW NOISE AGC AMPLIFIER WITH 80 dB GAIN 
RANGE 
Figure 37 provides an example of the ease with which the 
AD600 can be connected as an AGC amplifier. A1 and A2 are 
cascaded, with 6 dB of attenuation introduced by the 100 Ω 
resistor R1, while a time constant of 5 ns is formed by C1 and 
the 50 Ω of net resistance at the input of A2. This has the dual 
effect of lowering the overall gain range from 0 dB to 80 dB to 
6 dB to 74 dB and introducing a single-pole, low-pass filter 
with a 3 dB frequency of about 32 MHz. This ensures stability 
at the maximum gain for a slight reduction in the overall 
bandwidth. The Capacitor C4 blocks the small dc offset voltage 
at the output of A1 (which may otherwise saturate A2 at its 
maximum gain) and introduces a high-pass corner at about 
8 kHz, useful in eliminating low frequency noise and spurious 
signals that can be present at the input. 
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
REF
A1
A2
+
–
+
–
AD600
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
C1LO
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
R1
100
RF
INPUT
C4
0.1μF
C1
100pF
+5V
R3
46.4k
R4
3.74k
C3
15pF
AD590
+5V
Q1
2N3904
FB
FB
+5V
–5V
+5V DEC
–5V DEC
0.1μF
0.1μF
POWER SUPPLY
DECOUPLING NETWORK
RF
OUTPUT
+5V DEC
–5V DEC
+
–
V
PTAT
V
G
′
300μA
(AT 300K)
C2
1μF
R2
806
1%
0
Figure 37. This Accurate HF AGC Amplifier Uses Three Active Components