ETC AB-054

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By Jerald Graeme (602) 746-7412
clamps output eO by diverting feedback current away from
R2. This occurs when eO reaches a level sufficient to turn on
either Q1 or Q2. Then, a transistor collector current absorbs
any additional signal current supplied through R1. No further
signal current reaches R2 and this limits the level of eO.
Clamping amplifiers limit signal magnitude to protect following circuitry from input overload. The clamping amplifier shown here produces limit levels that track the power
supply levels selected in any given test configuration. Conventional clamping amplifiers set limit levels referenced to
ground, rather than the supply levels, and restrict signal
swing to the minimum available under minimum supply
conditions. However, the clamping amplifier shown automatically adapts the clamp levels to supply changes, maximizing the allowable signal swing.
Zener diodes DZ1 and DZ2 primarily determine the power
supply and clamp level relationships. These zeners establish
voltage levels with fixed separations from the supply voltages. Diode DZ1 biases the base of Q1 at V– +VZ1, and DZ2 sets
the base of Q2 at V+ –VZ2. These base biases set the clamp
transistors for turn on at specific clamp levels. A negative
going eO turns on transistor Q1 when eO reaches a level of VL–
= V– +VZ1 – VBE1 – VD1. Then, Q1 conducts current through
its collector, diverting any additional feedback current away
from R2. Any further increase in ei magnitude simply supplies more current to Q1 rather than to R2. This holds the
circuit output voltage at the turn-on level VL–. Similarly, a
positive going eO turns on Q2 when eO reaches the positive
limit of VL+ = V+ – VZ2 + VBE2 + VD2.
Traditional clamping amplifiers produce output voltage limits referenced to common. However, the input overload
levels of most circuits depend upon voltage levels referenced to the power supplies. There, the internal bias voltage
requirements of a circuit define minimum voltage separations between the signal and the power supply levels. This
references the overload levels to the supplies, rather than to
ground. Ground-referenced limits adequately accommodate
these bias requirements where the power supply levels
remain relatively fixed. Then, ground-referenced limits simply subtract fixed amounts from the worst-case, low supply
With the components shown, VZ = 6.2V and VBE and VD =
0.6V producing 10V output limiting for 15V supplies. Tolerance variations in the component voltages introduce a
400mV worst-case error to the clamp voltages. No significant clamp-level error results from the precision OPA77
However, power supply levels vary greatly in test systems
where control signals set the supply levels. Then, the worstcase setting of ground-referenced limits often sacrifices
operation in otherwise safe signal ranges. The clamping
amplifier shown adapts to supply variations by referencing
the clamp trip points to the supplies rather than to ground.
Higher supply voltages then produce higher clamp levels.
This avoids lost signal range by adapting the clamping limits
to the varying supply versus input capabilities of the following circuit.
However, the clamp circuit adds gain in the feedback loop,
compromising feedback stability. When one of the clamp
transistors conducts, it acts as a common-base transistor in
the feedback loop. This adds a gain of (R1 || R2)/RE to the
open-loop gain of the amplifier. Here, RE represents the
impedance of the transistor’s emitter circuit and this impedance is quite low. The emitter circuit impedance includes the
dynamic emitter impedance of the transistor and the forward
impedance of the diode. Feedback analysis1 shows that this
added gain shifts the net open-loop gain upward, exposing a
region of two-pole response roll off. Lower closed-loop
gains encounter this stability-compromising region. For those
cases, the capacitor shown rolls off the load impedance of
the common-base transistors, removing the added gain at
high frequencies.
Basically, the circuit shown consists of an inverting amplifier formed with the op amp, R1 and R2. The remaining
components produce the clamping action, provide breakdown protection, and phase compensate the circuit. The
zener diodes, transistors and R3 establish the basic clamp
reference voltages. Diodes D1 and D2 protect the transistors
from emitter-base breakdown and the capacitor compensates
the feedback stability complicated by the clamp. The circuit
1993 Burr-Brown Corporation
Printed in U.S.A. April, 1993
D1, D2:1N4154
DZ1, DZ2:1N4627
VL+ = V+ – VZ2 + VBE2 + VD2 = 10V
VL– = V– + VZ1 – VBE1 – VD1 = –10V
FIGURE 1. The limit levels of a clamping amplifier track the power supply levels when zener diodes establish fixed voltage
differences between the two levels.
1. Graeme, J. G., Feedback Plots Define Op Amp AC
Performance, Burr-Brown Application Bulletin AB-028,
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any BURR-BROWN product for use in life support devices and/or systems.