kh561 Fairchild Semiconductor, kh561 Datasheet - Page 10

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

DATA SHEET
An example calculation for the circuit in Figure 1 using
typical 25°C DC error terms and R
yields:
10
output with the same gain as the input signal, while the
inverting current errors have a gain of simply (R
the output voltage (neglecting the R
Output DC Offset:
The DC error terms shown in the specification listing
along with the model of Figure 5 may be used to estimate
the output DC offset voltage and drift. Each term shown
in the specification listing can be of either polarity. While
the equations shown below are for output offset voltage,
the same equation may be used for the drift with each
term replaced by its temperature drift value shown in the
specification listing.
Recall that the source impedance, R
terminating and signal source impedance and that the
actual DC level to the load includes the voltage divider
between R
as for all current feedback amplifiers, the non-inverting
and inverting bias currents do not track each other in
either magnitude or polarity. Hence, there is no meaning
in an offset current specification, and source impedance
matching to cancel bias currents is ineffective.
Noise Analysis:
Although the DC error terms are in fact random, the cal-
culation shown above assumes they are all additive in a
worst case sense. The effect of all the various noise
sources are combined as a root sum of squared terms to
get an overall expression for the spot noise voltage. The
circuit of Figure 8 shows the equivalent circuit with all the
various noise voltages and currents included along with
their gains to the output.
V
V
where: I
os
o
DC
=
=
[
(
(
I
bn
5 A 25
µ
I
V
bn
bi
⋅
io
R
o
≡
⋅
≡
s
and R
≡
inverting bias current
±
non inverting bias current
input offset voltage
Ω
V
io
±
−
L
)
2.0mV 10 10 A 360
. Also note that for the KH561, as well
⋅
attentuation between R and R



1
+
R
)
f
R
−
g
R
±
o

 ±

µ
I
o
1/ 2
bi
s
↑
(
s
to R
(
, includes both the
= 25Ω, R
R
= ±
f
L
Ω L
−
12.4mV
o
attenuation).
R
)
]
o
)
L
f
- R
= 50Ω
L
o
) to
√4kTR
where:
e
i
i
To get an expression for the equivalent output noise volt-
age, each of these noise voltage and current terms must
be taken to the output through their appropriate gains
and combined as the root sum of squares.
Where the 4kT(R
power of R
It is often more useful to show the noise as an equivalent
input spot noise voltage where every term shown above
is reflected to the input. This allows a direct measure of
the input signal to noise ratio. This is done by dividing
every term inside the radical by the signal voltage gain
squared. This, and an example calculation for the circuit
of Figure 1, are shown below. Note that R
neglected in this calculation.
e
e
ni
i
ni
o
n
– inverting input current noise
4
4
4
4
– non-inverting input current noise
kTR
kT R
kT R
kTR
=
=
– non-inverting input voltage noise
√
s
(
/
*
4kT
s
o
(
e
R
f
e
g
ni
g
−
−
−
ni
2
R
−
noise
Figure 8: Equivalent Noise Model
2
source resis
R
output resistor voltage noise
*
g
s
+
noise current
gain settling resistor
+
o
and R
(
)
(
i R
ni s
i R
*
*
−
ni s
voltage noise
i
i
feedback resistor
ni
i
f
)
e
f
- R
*
)
2
ni
- R
2
R
+
g
+
o
tan
4
o
4
) A
.
kTR
Classical
op-amp
kTR
+
-
4
+
ce voltage
kT R
v
R
4
s
term is the combined noise
kT R
f
s
(
- R
√4kT(R
+
)
A
A
(
i R
o
f
i
v
2
v
−
(
2
f
R
−
f
*
+
A
f
o
- R
R
i
−
v
)
i
2
o
2
o
R
REV. 1A February 2001
+
)
)
(
R
o
A
4
)
f
kTR
A
v
2
−
*
v
+
+
R
2
√4kTRV
4
L
o
Gain to e
o
L
kTR
)
R
R
R
2
may be
A
o
f
f
L
A
A
KH561
v
- R
- R
1
1
o
R
o
v
v
s
o
o
e
o
o