AD829 Data Sheet
Rev. I | Page 12 of 20
THEORY OF OPERATION
The AD829 is fabricated on the Analog Devices, Inc., proprietary
complementary bipolar (CB) process, which provides PNP and
NPN transistors with similar f
T
s of 600 MHz. As shown in
Figure 35, the AD829 input stage consists of an NPN differential
pair in which each transistor operates at a 600 µA collector current.
This gives the input devices a high transconductance, which in
turn gives the AD829 a low noise figure of 2 nV/√Hz at 1 kHz.
Figure 35. Simplified Schematic
The input stage drives a folded cascode that consists of a fast pair of
PNP transistors. These PNPs drive a current mirror that provides a
differential-input-to-single-ended-output conversion. The high
speed PNPs are also used in the current-amplifying output stage,
which provides a high current gain of 40,000. Even under heavy
loading conditions, the high f
T
s of the NPN and PNPs, produced
using the CB process, permit cascading two stages of emitter
followers while maintaining 60 phase margin at closed-loop
bandwidths greater than 50 MHz.
Two stages of complementary emitter followers also effectively
buffer the high impedance compensation node (at the C
COMP
pin)
from the output so that the AD829 can maintain a high dc open-
loop gain, even into low load impedances (92 dB into a 150 Ω
load and 100 dB into a 1 kΩ load). Laser trimming and PTAT
biasing ensure low offset voltage and low offset voltage drift,
enabling the user to eliminate ac coupling in many applications.
For added flexibility, the AD829 provides access to the internal
frequency compensation node. This allows users to customize the
frequency response characteristics for a particular application.
Unity-gain stability requires a compensation capacitance of 68 pF
(Pin 5 to ground), which yields a small signal bandwidth of
66 MHz and slew rate of 16 V/µs. The slew rate and gain
bandwidth product varies inversely with compensation
capacitance. Table 4 and Figure 37 show the optimum
compensation capacitance and the resulting slew rate for
a desired noise gain.
For gains between 1 and 20, choose C
COMP
to keep the small signal
bandwidth relatively constant. The minimum gain that will still
provide stability depends on the value of the external
compensation capacitance.
An RC network in the output stage (see Figure 35) completely
removes the effect of capacitive loading when the amplifier
compensates for closed-loop gains of 10 or higher. At low
frequencies, and with low capacitive loads, the gain from the
compensation node to the output is very close to unity. In this case,
C is bootstrapped and does not contribute to the compensation
capacitance of the device. As the capacitive load increases, a pole
forms with the output impedance of the output stage, which
reduces the gain, and subsequently, C is incompletely
bootstrapped. Therefore, some fraction of C contributes to the
compensation capacitance, and the unity-gain bandwidth falls.
As the load capacitance is further increased, the bandwidth
continues to fall, and the amplifier remains stable.
EXTERNALLY COMPENSATING THE AD829
The AD829 is stable with no external compensation for noise
gains greater than 20. For lower gains, two different methods of
frequency compensating the amplifier can be used to achieve
closed-loop stability: shunt and current feedback compensation.
SHUNT COMPENSATION
Figure 36 and Figure 37 show that shunt compensation has an
external compensation capacitor, C
COMP
, connected between the
compensation pin and ground. This external capacitor is tied in
parallel with approximately 3 pF of internal capacitance at the
compensation node. In addition, a small capacitance, C
LEAD
, in
parallel with resistor R2, compensates for the capacitance at the
inverting input of the amplifier.
Figure 36. Inverting Amplifier Connection Using External Shunt
Compensation
Figure 37. Noninverting Amplifier Connection Using External Shunt
Compensation
Table 4 gives the recommended C
COMP
and C
LEAD
values, as well
as the corresponding slew rates and bandwidth. The capacitor
values were selected to provide a small signal frequency response
with <1 dB of peaking and <10% overshoot. For Table 4, ±15 V
00880-035
–IN+IN
1.2mA
OFFSET NULL
C
12.5pF
R
500Ω
OUTPUT
+V
S
–V
S
C
COMP
15Ω
15Ω
00880-036
2
3
7
6
4
–
+
AD829
+V
S
–V
S
0.1µF
0.1µF
5
C
COMP
50Ω
50Ω
COAX
CABLE
R1
C
LEAD
R2
V
OUT
1kΩ
V
IN
00880-037
3
2
7
6
4
+
–
AD829
+V
S
–V
S
0.1µF
0.1µF
C
LEAD
5
C
COMP
50Ω
50Ω
CABLE
V
OUT
R2
R1
1kΩ
V
IN
