kh561 Fairchild Semiconductor, kh561 Datasheet - Page 8

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

DATA SHEET
recognize that [taking V positive]
V
solving for V from two directions
V
solving for i
i
V
and, substituting for V and i
V
pulling an
note that A
8
then
A
Note again that if R
considerably. Also, if G were very large the voltage gain
expression would reduce to the familiar non-inverting op
amp gain equation. These two performance equations,
shown below, provide a means to derive the design equa-
tions for R
put impedance.
err
R
Performance Equations
A
o
o
−
−
v
=
o
v
=
=
=
=
≡
V
=
= +
V 1
V
V
(
V
V
−
G 1 R
i
1
i
i
R
o
i
−
−




+
f
+
= +
i
G
Gi
+
R
(
+
err
R
R
R
G 1 R
1
f
)
v
Figure 4: Voltage Gain Derivation
err
R
g
V
f
(
g
+ +
err
f
and R
−
G 1 R
+
R
i
R
i
1
= +






g
out of the fraction
R
R



from this
V R
GR
i
i
+
G
R
1
)
1
+
g
=
i
f
=
G
+
R
f
R
+ +
R
)
(
0






f
g
G 1 i
g
R
g
i
R
R
1
R
i
−
G 1
i
−
+
g
given a desired no load gain and out-
g
g
f
R
+
G
f
R
R
+ +
i
R



R
+
−



R
i
f
i i
−
= 0 this expression would simplify
)
i
i
G 1
R
g
i
R
err
R
G
i






+
R




R
i
f
R
g
i



err






g
in the original V expression
R
R
Design Equations
f
g
=
=
(
R
G
A
f
+
v
−
1
−
R
)
1
R
o
o
o
−
A R
v
i
Equivalent Model
Given that the physical feedback and gain setting
resistors have been determined in accordance with the
design equations shown above, an equivalent model may
be created for the gain to the load where the
amplifier block is taken as a standard op amp. Figure 5
shows this analysis model and the resulting gain
equation to the load.
substituting in for R and R with their design
equation yields
This model is used to generate the DC error and noise
performance equations. As with any equivalent model,
the primary intent is to match the external terminal
characteristics recognizing that the model distorts the
internal currents and voltages. In this case, the model
would incorrectly predict the output pin voltage swing for
a given swing at the load. But it does provide a simplified
means of getting to the external terminal characteristics.
External Compensation Capacitor (C
As shown in the test circuit of Figure 1, the KH561 requires
an external compensation capacitor from the output to
pin 19. The recommended values described here assume
that a maximally flat frequency response into a matched
load is desired.
the desired value of output impedance and to a lesser
degree on the desired gain. Note from Figure 2, the
simplified internal schematic, that the actual total
compensation (C
the internal 10pF from pin 19 to the compensation nodes.
The total compensation (C
shown below.
V
V
C
C
V
V
o
o
1
i
i
t
=
=
=
=
1
300



A
R
1
+
v
V
o
+
(
i
R
0.02 C
R



C
L
1
f
R
R
1
R
+
−
−
L
g
)
g
R
R
Figure 5: Equivalent Model
2.0
R
o
o
g
1
Classical
op-amp
+
-



=



t
) is the series combination of C
pF total compensation
R
f
The required C
A (gain to load)
pF intermediate equation
R
L
L
f
R
+
- R
L
R
o
o
g
t
) is developed in two steps as
R
x
o
varies widely with
REV. 1A February 2001
x
)
R
L
V
o
KH561
x
and