kh561 Fairchild Semiconductor, kh561 Datasheet - Page 7

• Manufacturer Part Number: kh561
• Description: Wideband, Low Distortion Driver Amplifier
• Manufacturer: Fairchild Semiconductor

KH561
part, relatively constant performance over supply voltage
is achieved. A current sense in the error current leg of
the 10X current mirror feeds back to the bias current
setup providing a current shutdown feature when the
output current approaches 250mA.
Developing the Performance Equations
The KH561 is intended to provide both a controllable
voltage gain from input to output as well as a controllable
output impedance. It is best to treat these two operations
separately with no load in place. Then, with the no-load
gain and output impedance determined, the gain to the
load will simply be the no-load gain attenuated by the
voltage divider formed by the load and the equivalent
output impedance.
Figure 3 steps through the output impedance develop-
ment using an equivalent model of Figure 2. Offering an
equivalent, non-zero, output impedance into a matched
load allows the KH561 to operate at lower internal volt-
age swings for a given desired swing at the load. This
allows higher voltage swings to be delivered at the load
for a given power supply voltage at lower distortion levels
than an equivalent op amp needing to generate twice the
voltage swing actually desired at the matched load. This
improved distortion is specified and tested over a wide
range as shown in the specification listing.
REV. 1A February 2001
V
i
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Figure 2: Simplified Circuit Diagram
I
8
I
bias
bias
V -
R
Q1
Q2
g
+
-
+V
-V
Current Limit
Current Limit
R
X1
CC
CC
i
R
f
i
f
Q3
Q4
Gi
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R
+V
-V
21
10X Current Mirror
10X Current Mirror
4
g
CC
CC
19
5pF
5pF
R
f
C
x
R
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23
I
I
o
o
l
o
R
V
o
o
V
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i
I
R
note that R
Get both V
using:
V
V
V
and
then
Note that the R
R
go to infinity, the output impedance would go to 0. This
would be the normal op amp topology with a very high
internal gain. The KH561 achieves a non-zero R
setting the internal forward gain to be a low, well
controlled, value.
Developing the No-Load Gain Expression
Taking the output impedance expression as one con-
straint setting the external resistor values, we now need
to develop the no-load voltage gain expression from the
non-inverting input to the output as the other constraint.
Figure 4 shows the derivation of the no load gain.
f
o
o
o
o
−
i
=
=
= 0. Also note that if the forward current gain were to
=
=
=
i
≡
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Gi
i
V
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i
V
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−
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+
Figure 3: Output Impedance Derivation
+




R and
V -
R
R
R
=
V
+ =
i R
i
g
f
f
−
g
R
o
o
i
+
f
-
+
and I
f
=
R
f
R 1
=
G 1
+
=
i
i
i
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R 1
G 1
=
+ +
i



R
R
X1
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o
o
+
0






f




i
+
into terms of just the error current, i
1
G 1
expression simplifies considerably if




R
+
R
R
+
R
R
R
+ + +
g
R
i
f
R
g
f
R
R
i
+



g
g
i
f




R 1






R
R
Gi
f
g



i
err




+
R
R
g
i







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DATA SHEET
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by
7
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