Datasheet

AD600/AD602
LOW NOISE AGC AMPLIFIER WITH 80 dB GAIN
RANGE
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.
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 
Resistor R1, while a time constant of 5 ns is formed by C1 and
the 50  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 C4 capacitor 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.
Table 4. Measured Preamplifier Performance
Measurement Value Unit
Gain (f = 30 MHz) 6 dB
Bandwidth (−3 dB) 250 MHz
Input Signal for 1 dB Compression 1 V p-p
Distortion
V
IN
= 200 mV p-p HD2 0.27 %
HD3 0.14 %
V
IN
= 500 mV p-p HD2 0.44 %
HD3 0.58 %
System Input Noise 1.03 nV/√Hz
Spectral Density (NSD)
(Preamp Plus X-AMP)
Input Resistance 1.4 kΩ
Input Capacitance 15 pF
Input Bias Current ±150 µA
Power Supply Voltage ±5 V
Quiescent Current 15 mA
Rev. F | Page 17 of 32
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
100Ω
C4
0.1µF
C1
100pF
+5
V
R3
46.4kΩ
R4
3.74kΩ
RF
INPUT
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
806Ω
1%
00538-035
Figure 37. This Accurate HF AGC Amplifier Uses Three Active Components