廬
APPLICATION BULLETIN
TUNING IN AMPLIFIERS
By Bonnie Baker
Mailing Address: PO Box 11400, Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 鈥?Tel: (520) 746-1111
Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Product Info: (800) 548-6132 鈥?Internet: www.burr-brown.com/ 鈥?FAXLine: (800) 548-6133
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Have you ever had the experience of designing an analog
gain block with an amplifier that is specified to be unity gain
stable only to find that it is oscillating out of control in your
circuit? Or have you ever replaced a stable voltage feedback
amplifier with a current feedback amplifier to find that the
current feedback amplifier immediately oscillates when
placed in the amplifier socket? Oscillation problems are a
nuisance to track down, particularly if there is no clear game
plan. When troubleshooting an oscillating amplifier circuit,
several questions come to mind, such as, has the feedback
loop been properly configured to insure stability? Have the
effects of loading the output of the amplifier been consid-
ered? Are the by-pass capacitors properly positioned on the
board in respect to the amplifier? Is the PCB layout executed
properly to avoid the ill effects of trace parasitics and cross-
talk? This simple check-list with some general knowledge
about what determines amplifier stability, or lack thereof,
can help the designer identify oscillation problems and
implement effective, stable solutions.
DESIGNING AROUND THE AMPLIFIER
When beginning the troubleshooting process, the first step
the designer should take determines whether or not the
resistors, capacitors and inductors that are used around the
amplifier鈥檚 input, feedback and output are appropriately
(a)
applied. The selection of these components is dependent on
first and foremost the type of amplifier that is being used,
i.e., voltage feedback as opposed to current feedback ampli-
fier. Once the amplifier type is known the stability equations
quickly fall out of simple calculations. Take, as an example,
an amplifier configured in a non-inverting circuit as shown
in Figure 1a. The amplifier is configured in a non-inverting
circuit where the low frequency gain is (1 + R
F
/R
IN
). Here,
R
F
and R
IN
are the low frequency equivalent impedance of
Z
F
and Z
IN
. In the case of Figure 1a, a voltage feedback
amplifier is used in the circuit. A current feedback amplifier
could be used instead while still achieving good circuit
stability.
VOLTAGE FEEDBACK AMPLIFIER ANALYSIS
The voltage feedback amplifier is the most prolific amplifier
on the market. Dependent on the characteristics of the
specific amplifier, they are used in high speed as well as
precision applications. Since the preferred frame of refer-
ence for most analog designers is the voltage feedback
amplifier, the stability analysis begins with the topology
shown in Figure 1a. This simplified block diagram illus-
trates many of the key characteristics needed in a frequency
analysis of the voltage feedback amplifier. Starting with the
input segment of the amplifier, the inputs to the voltage
(b)
V
OUT
(s)
C
CM
3
4
5
6
Z
IN
Z
F
=
(1 + Z
F
/Z
IN
)
1 + (1 + Z
F
/Z
IN
)/A
OL
(s)
G
N
V
IN
(s)
C
C
鈥?/div>
V
ERR
V
IN
+
C
CM
C
DIFF
+
Gain
V
OUT
A
OL
(s)
A
OL
~20dB/decade
V
ERR
OPA650
G
N1
G
N2
A
OL
(s) =
G
DC
1 + r
O
C
C
s
, s = jw
V
OUT
= A
OL
(s) 鈥?V
ERR
V
IN
鈥?V
ERR
Z
IN
=
V
OUT
鈥?(V
IN
鈥?V
ERR
)
Z
F
f
1
f
2
Frequency
FIGURE 1a). The Model of a Voltage Feedback Amplifier Configured in a Non-Inverting Closed-Loop Configuration.
1b). Bode Plot Response of Various Closed-Loop Non-Inverting Systems Using a Voltage Feedback Amplifier.
漏
1996 Burr-Brown Corporation
AB-105
Printed in U.S.A. October, 1996
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