AD600 Analog Devices, Inc., AD600 Datasheet - Page 17

• Manufacturer Part Number: AD600
• Description: Dual, Low Noise, Wideband Variable Gain Amplifiers
• Manufacturer: Analog Devices, Inc.

This is a Class AB amplifier. As V
direction, Q1 conducts more heavily and its r
while Q2 increases. Conversely, increasingly negative values of
V
The design is chosen such that the net emitter resistance is
essentially independent of the instantaneous value of V
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
System Input Noise
Input Resistance
Input Capacitance
Input Bias Current
Power Supply Voltage
Quiescent Current
IN
V
V
Spectral Density (NSD)
(Preamp plus X-AMP)
INPUT
result in the r
IN
IN
= 200 mV p-p
= 500 mV p-p
RF
e
of Q2 decreasing, while the r
C1LO
A1LO
GAT1
GAT2
A2LO
C2LO
A1HI
A2HI
+5V
R3
46.4kΩ
R4
3.74kΩ
1
2
3
4
5
6
7
8
+
–
–
+
AD600
IN
increases in a positive
A1
A2
REF
Figure 37. This Accurate HF AGC Amplifier Uses Three Active Components
HD2
HD3
HD2
HD3
16
15
14
13
12
11
10
9
e
e
of Q1 increases.
A2CM
C1HI
VPOS
VNEG
A2OP
C2HI
A1CM
A1OP
becomes lower
Value
6
250
1
0.27
0.14
0.44
0.58
1.03
1.4
15
±150
±5
15
+5V DEC
–5V DEC
IN
0.1µF
Unit
dB
MHz
V p-p
%
%
%
%
nV/√Hz
kΩ
pF
μA
V
mA
C4
,
Rev. E | Page 17 of 28
100Ω
R1
100pF
C1
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.
V
G
´
15pF
1µF
C3
C2
AD590
806Ω
1%
R2
+5V
+
–
V
Q1
2N3904
PTAT
300µA
(AT 300K)
RF
OUTPUT
+5V DEC
–5V DEC
DECOUPLING NETWORK
POWER SUPPLY
AD600/AD602
FB
FB
–5V
+5V
0.1µF
0.1µF