TL/H/8455
Fast Compensation Extends Power Bandwidth LB-4
National Semiconductor
Linear Brief 4
April 1969
Fast Compensation
Extends Power Bandwidth
In all IC operational amplifiers the power bandwidth de-
pends on the frequency compensation. Normally, compen-
sation for unity gain operation is accompanied by the lowest
power bandwidth. A technique is presented which extends
the power bandwidth of the LM101A for non-inverting gains
of unity to ten, and also reduces the gain error at moderate
frequencies.
In order to achieve unconditional stability, an operational
amplifier is rolled off at 6 dB per octave, with an accompa-
nying 90 degrees of phase shift, until a gain of unity is
reached. Unity gain in most monolithic operational amplifi-
ers is limited to 1 MHz, because the lateral PNP’s used for
level shifting have poor frequency response and exhibit ex-
cess phase shift at frequencies above 1 MHz. Hence, for
stable operation, the closed loop bandwidth must be less
than 1 MHz where the phase shift remains below 180 de-
grees.
For high closed loop gains, less severe frequency compen-
sation is necessary to roll the open loop gain off at 6 dB per
octave until it crosses the closed loop gain. The frequency
where it crosses must, as previously mentioned, be less
than 1 MHz. For closed loop gains between 1 and 10, more
frequency compensation must be used to insure that the
open loop gain has been rolled off soon enough to cross the
closed loop gain before 1 MHz is reached.
The power bandwidth of an operational amplifier depends
on the current available to charge the frequency compensa-
tion capacitors. For unity gain operation, where the compen-
sation capacitor is largest, the power bandwidth of the
LM101A is 6 kHz.
Figure 1
shows an LM101A with unity gain
compensation and
Figure 3
shows the open loop gain as a
function of frequency.
TL/H/8455–1
FIGURE 1. LM101A with Standard Frequency
Compensation
A two-pole frequency compensation network, as shown in
Figure 2
, provides more than a factor of two improvement in
power bandwidth and reduced gain error at moderate fre-
quencies. The network consists of a 30 pF capacitor, which
sets the unity gain frequency at 1 MHz, along with a 300 pF
capacitor and a 10k resistor. By dividing the AC output volt-
TL/H/8455–2
FIGURE 2. LM101A with Frequency Compensation to
Extend Power Bandwidth
age with the 10k resistor and 300 pF capacitor, there is less
AC voltage across the 30 pF capacitor and less current is
needed for charging. Since the voltage division is frequency
sensitive, the open loop gain rolls off at 12 dB per octave
until a gain of 20 is reached at 50 kHz. From 50 kHz to
1 MHz the 10k resistor is larger than the impedance of the
300 pF capacitor and the gain rolls off at 6 dB per octave.
The open loop gain plot is shown in
Figure 3
. To insure
sufficient drive to the 300 pF capacitor, it is connected to
the output, Pin 6, rather than Pin 8. With this frequency com-
pensation method, the power bandwidth is typically
15– 20 kHz as a follower, or unity gain inverter.
TL/H/8455–3
FIGURE 3. Open Loop Response for Both Frequency
Compensation Networks
This frequency compensation, in addition to extending the
power bandwidth, provides an order of magnitude lower
gain error at frequencies from DC to 5 kHz. Some applica-
tions where it would be helpful to use the compensation are:
differential amplifiers, audio amplifiers, oscillators, and ac-
tive filters.
C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
LB-4 Fast Compensation Extends Power Bandwidth
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