Overdriving the Input Stage
Sustained input differential voltages greater than 3.4 V should be
avoided as the input transistors may be damaged. Input clamp
diodes are recommended if the possibility of this condition exists.
The voltages at the collectors of the input pairs are set to 200 mV
from the power supply rails. This allows the amplifier to remain
in linear operation for input voltages up to 500 mV beyond the
supply voltages. Driving the input common-mode voltage beyond
that point will forward bias the collector junction of the input
transistor, resulting in phase reversal. Sustaining this condition
for any length of time should be avoided as it is easy to exceed
the maximum allowed input differential voltage when the amplifier
is in phase reversal.
Output Stage, Open-Loop Gain, and Distortion Versus
Clearance from Power Supply
The amplifier features a rail-to-rail output stage. The output
transistors operate as common emitter amplifiers, providing the
output drive current as well as a large portion of the amplifier’s
Used as a unity gain follower, the amplifier output will exhibit
more distortion in the peak output voltage region around
VCC –0.7 V. This unusual distortion characteristic is caused by
the input stage architecture and is discussed in detail in the
section covering Input Stage Operation.
Output Overdrive Recovery
Output overdrive of an amplifier occurs when the amplifier
attempts to drive the output voltage to a level outside its normal
range. After the overdrive condition is removed, the amplifier must
recover to normal operation in a reasonable amount of time. As
shown in Figure 41, the amplifier recovers within 100 ns from
negative overdrive and within 80 ns from positive overdrive.
RF = RG = 2k⍀
Figure 40. Output Stage Simplified Schematic
The output voltage limit depends on how much current the
output transistors are required to source or sink. For applica-
tions with very low drive requirements (a unity gain follower
driving another amplifier input, for instance), the amplifier
typically swings within 20 mV of either voltage supply. As the
required current load increases, the saturation output voltage
will increase linearly as ILOAD ϫ RC, where ILOAD is the required
load current and RC is the output transistor collector resistance.
For the amplifier, the collector resistances for both output tran-
sistors are typically 25 W. As the current load exceeds the rated
output current of 15 mA, the amount of base drive current
required to drive the output transistor into saturation will reach
its limit, and the amplifier’s output swing will rapidly decrease.
The open-loop gain of the amplifier decreases approximately
linearly with load resistance and also depends on the output
voltage. Open-loop gain stays constant to within 250 mV of the
positive power supply, 150 mV of the negative power supply
and then decreases as the output transistors are driven further
The distortion performance of the amplifiers differs from
conventional amplifiers. Typically an amplifier’s distortion
performance degrades as the output voltage amplitude increases.
VS = ؎2.5V
VIN = ؎2.5V
RL = 1k⍀ TO GND
Figure 41. Overdrive Recovery
Driving Capacitive Loads
Capacitive loads interact with an amplifier’s output impedance
to create an extra delay in the feedback path. This reduces circuit
stability and can cause unwanted ringing and oscillation. A given
value of capacitance causes much less ringing when the amplifier
is used with a higher noise gain.
The capacitive load drive of the amplifier can be increased by
adding a low valued resistor in series with the capacitive load.
Introducing a series resistor tends to isolate the capacitive load
from the feedback loop, thereby diminishing its influence.
Figure 42 shows the effect of a series resistor on capacitive drive
for varying voltage gains. As the closed-loop gain is increased, the
larger phase margin allows for larger capacitive loads with less
overshoot. Adding a series resistor at lower closed-loop gains
accomplishes the same effect. For large capacitive loads, the
frequency response of the amplifier will be dominated by the
roll-off of the series resistor and capacitive load.
VS = 5
WITH 30% OVERSHOOT
RS = 5⍀
RS = 0⍀
RS = 20⍀
RS = 0⍀, 5⍀
RS = 20V
CLOSED-LOOP GAIN – V/V
Figure 42. Capacitive Load Drive vs. Closed-Loop Gain