廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
FEEDBACK CIRCUIT CLAMPS PRECISELY
by Jerald Graeme, (602) 746-7412
A limiter circuit consisting of an input buffer (A
1
), an
output-scaling amplifier (A
2
), two zener diodes (Z
1
and Z
2
),
and several other components can supply sharp, precise,
bipolar clamp levels with continuous variable control, from
0 to
鹵11V.
See Figure 1. A feedback loop enclosing the
amplifiers and zeners generates the high clamping accuracy.
Within the limit range of the clamp (鹵V
L
), the zener diodes
are off, and A
2
feeds back its output to the inverting input of
A
1
through R
4
. At the same time A
1
drives A
2
through the
voltage divider R
V
. The feedback forces the inverting input
of op amp A
1
to equal E
I
at the noninverting input terminal.
The circuit forces the inverting input of A
2
also to follow E
I
.
There鈥檚 no signal voltage drop across R
4
, because no current
can flow from it into A
2
鈥檚 inverting input. Consequently, the
noninverting input of A
2
, which defines the potentiometer
output at feedback equilibrium, must also track E
I
. A resistor
voltage divider can replace the control potentiometer R
V
in
fixed-level limiting applications.
Amplifier A
2
then delivers an output:
E
O
= (1 + R
3
/R
2
) E
I
when
鈥揤
L
< E
O
< V
L
and
V
L
= x [(1 + R
3
/R
2
)] (V
Z
+ V
F
)
where x is the setting fraction of R
V
, and V
Z
and V
F
are the
zener and forward voltages, respectively. The overall circuit
response, then, is simply that of a voltage amplifier when the
output signal is within the limit boundaries.
Amplifier A
1
generates small deviations from an ideal re-
sponse because A
2
鈥檚 circuit gain (1 + R
3
/R
2
) amplifies any
offset voltage and noise from A
1
. Similarly, this loop gain
mitigates the clamping error by sharpening its clamping
response. The zener drive increases during the transition to
the clamping state.
90
1k鈩?/div>
R
4
C
2200pF
1k鈩?/div>
R
1
A
1
E
I
1/2
OPA2111
Z
1
1N4626
Z
2
V
L
E
O
E
I
鈥揤
L
E
O
= (1 + R
3
/R
2
) E
I
,
for 鈥揤
L
< E
O
< V
L
V
L
= x(1 + R
3
/R
2
)(V
Z
+ V
F
)
R
V
20k鈩?/div>
10k鈩?/div>
R
3
A
2
1/2
OPA2111
R
2
13k鈩?/div>
E
O
FIGURE 1. Amplifier A
1
Buffers and Amplifier A
2
Scales Input Signals Under Feedback Control. Zener diodes and a
potentiometer or voltage divider in the feedback loop supply a continuously variable bipolar-clamping limit.
漏
1994 Burr-Brown Corporation
AB-090
Printed in U.S.A. January, 1994
Application Bulletin Number 90
AB-090相關(guān)型號PDF文件下載
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描述
廠商
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