Texas Instruments | LM118 Op Amp Slews 70 V/sec | Application notes | Texas Instruments LM118 Op Amp Slews 70 V/sec Application notes

Texas Instruments LM118 Op Amp Slews 70 V/sec Application notes
LM118,LM741
LM118 Op Amp Slews 70 V/?'sec
Literature Number: SNOA850
National Semiconductor
Linear Brief 17
September 1971
One of the greatest limitations of today’s monolithic op amps
is speed. With unity gain frequency compensation, general
purpose op amps have 1 MHz bandwidth and 0.3 V/µs slew
rate. Optimized compensation as well as feedforward compensation can improve op amp speed for some applications.
Specialized devices such as fast, unity-gain buffers are
available which provide partial solutions. This paper will
describe a new high speed monolithic amplifier that offers an
order of magnitude increase in speed with no loss in flexibility over general purpose devices.
Figure 1 shows a simplified schematic of the LM118. Transistors Q1 and Q2 are a conventional differential input stage
with emitter degeneration and resistive collector loads. Q3
and Q4 form the second stage which further amplify the
signal and level shift the signal towards V−. The collectors of
Q3 and Q4 drive a current inverter, Q10 and Q11 to convert
from differential to single ended. Q9, which has a current
source load for high gain, drives a class B output. The
collectors of the input stage and the base of Q9 are available
for offset balancing and external compensation.
The LM118 is constructed by the standard six mask monolithic process and features 15 MHz bandwidth and 70 V/µs
slew rate. It operates over a ± 5 to ± 18V supply range with
little change in speed. Additionally, the device has internal
unity-gain frequency compensation and needs no external
components for operation. However, unlike other internally
compensated amplifiers, external feedforward compensation
may be added to approximately double the bandwidth and
slew rate.
Frequency compensation is accomplished with three internal
capacitors. C1 rolls off on half the differential input stage so
that the high frequency signal path is single-ended. Also, at
high frequencies, the signal is fed forward around the lateral
PNP transistors by a 30 pF capacitor, C2. This eliminates the
excessive phase shift. Overall frequency response is then
set by capacitor, C3, which rolls off the amplifier at 6 dB/
octave. As previously mentioned feedforward compensation
for inverting applications can be applied to the base of Q9.
Figure 2 shows the open loop frequency response of an
LM118. Table 1 gives typical specifications for the new amplifier.
Design Concepts
In general purpose amplifiers the unity-gain bandwidth is
limited by the lateral PNP transistors used for level shifting.
The response above 2 MHz is so poor that they cannot be
used in a feedback amplifier. If the PNP transistors are used
for level shifting only at DC or low frequencies and the signal
is fed forward around the PNP transistors at high frequencies, wide bandwidth can be obtained without the excessive
phase shift of the PNP transistors.
LM118 Op Amp Slews 70 V/µsec
LM118
Op Amp Slews 70 V/µsec
00683102
FIGURE 2. Open Loop Voltage Gain as a
Function of Frequency for LM118
TABLE 1. Typical Specifications for the LM118
Input Offset Voltage
Input Bias Current
200 nA
Offset Current
20 nA
Voltage Gain
200k
Output Voltage Swing
± 11.5V
± 13V
Small Signal Bandwidth
15 MHz
Slew Rate
70 V/µs
Common Mode Range
00683101
2 mV
FIGURE 1. Simplified Circuit of the LM118
LB-17
© 2002 National Semiconductor Corporation
AN006831
www.national.com
LB-17
At high gains, or with high value feedback resistors R4 can
be quite low — but not less than 100Ω. When the LM118 is
used as a fast integrator, with a large feedback capacitor or
with low values of feedback resistance, R4 must be increased to 8 kΩ to ensure stability over a full −55˚C to
+125˚C temperature range.
Operating Configuration
Although considerable effort was taken to make the LM118
trouble free, high frequency amplifiers are more prone to
oscillations than low frequency devices such as the LM101A.
Care must be taken to minimize the stray capacitance at the
inverting input and at the output; however the LM118 will
drive a 100 pF load. Good power supply bypassing is also in
order — 0.1 µF disc ceramic capacitors should be used
within a few inches of the amplifier. Additionally, a small
capacitor is usually necessary across the feedback resistor
to compensate for unavoidable stray capacitance.
Figure 3 shows feedforward compensation of the LM118 for
fast inverting applications. The signal is fed from the summing junction to the output stage driver by C1 and R4.
Resistors R5, R6 and R7 have two purposes: they increase
the internal operating current of the output stage to increase
slew rate and they provide offset balancing. The current
boost is necessary to drive internal stray capacitance at the
higher slew rate. Mismatch of the external resistors can
cause large voltage offsets so offset balancing is necessary.
For supply voltages other than ± 15V, R5 and R6 should be
selected to draw about 500 µA from Pins 1 and 5.
One of the more important considerations for a high speed
amplifier is settling time. Poor settling time can cancel the
advantages of having high slew rate and bandwidth. For
example — an amplifier can have severe ringing after a step
input. A relatively long time is then needed before the output
voltage can be read accurately. Settling time is the time
necessary for the output to slew through a defined voltage
change and settle to within a defined error of its final output
voltage. Figure 4 shows optimized compensation for settling
to within 0.1% error. Typically the settling time is 800 ns for a
simple inverter circuit as shown. Settling time is, of course,
subject to operating conditions external to the IC such as
closed loop gain, circuit layout, stray capacitance and source
resistance. An optional offset balancing circuit, R3 and R4 is
included.
00683104
00683103
†Slew and settling time to 0.1% for a 10V step change is 800 ns.
†Slew rate typically 120 V/µs
FIGURE 4. Compensation for Minimum Settling† Time
FIGURE 3. Feedforward Compensation
for Greater Inverting Slew Rate†
The LM118 opens up new fields for IC operational amplifiers.
It is more than an order of magnitude faster than general
purpose amplifiers while retaining the ease of use features. It
is ideally suited for analog to digital converters, active filters,
sample and hold circuits and wide band amplification. Further, the LM118 has the same pin configuration as the
LM101A or LM741 and is interchangeable with these devices when speed is of prime concern.
When using feedforward resistor R4 should be optimized for
the application. It is necessary to have about 8 kΩ in the path
from the output of the amplifier through the feedback resistor
and through feedforward network to Pin 8 of the device. The
series resistance is needed to limit the bandwidth and prevent minor loop oscillation.
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LM118 Op Amp Slews 70 V/µsec
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