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
Value
6
250
1
HD2
HD3
HD2
HD3
0.27
0.14
0.44
0.58
1.03
Unit
dB
MHz
V p-p
%
%
%
%
nV/√Hz
A LOW NOISE AGC AMPLIFIER WITH 80 dB GAIN
RANGE
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.
1.4
15
±150
±5
15
kΩ
pF
μA
V
mA
+5V
R3
46.4kΩ
R4
3.74kΩ
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
C4
0.1µF
R1
100Ω
C1
100pF
C3
15pF
R2 +
806Ω V
PTAT
1% –
C2
1µF
+5V
V
G
´
16
AD590
C1LO
RF
INPUT
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
300µA
(AT 300K)
FB
Q1
2N3904
+5V DEC
+5V
1
2
3
4
+
–
A1
15
14
13
0.1µF
0.1µF
+5V DEC
–5V DEC
–5V DEC
FB
RF
OUTPUT
–5V
POWER SUPPLY
DECOUPLING NETWORK
00538-035
REF
5
12
11
8
+
7
–
6
A2
10
9
AD600
Figure 37. This Accurate HF AGC Amplifier Uses Three Active Components
Rev. E | Page 17 of 28
AD [ ANALOG DEVICES ]
