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CIRCUIT DESCRIPTION
The VCE —IC characteristics are based on the parameter
IB.
When
|B ts constant,
the VCE—IC
characteristics are
expressed by a single curve.
In a region where the VCE—IC
characteristics are saturated, IC is almost constant regardless
of variations
in VCE.
Namely
the circuit
assures
the
constant-current
characteristics.
When
a constant-current
circuit is used on the emitter side (source side) of the dif-
ferential amplifier, the CMRR
value will be improved.
If it
is used as an amplifier load, then it functions as a light load
and the current
can
be always constant
and sufficient.
With these advantages, it can be regarded as a high-imped-
ance circuit in terms of AC.
ae
ae
i
ed
Ww
LN
if
ie
ea
ies
Ed
ze
fe
No
E
jo~ VGE
(LOW VOLTAGE, LOW CURRENT
REGION)
7
ww
WAWAGR
ee eee
Collector
current
Voltage between
collector & emitter
le
——__|
L—AVce—|
|
| R= SVOE (large)
:
l
Ate (small)
|
{ + R= large
Voce
<Ic-VceE
Characteristics >
CMRR
The former is the result of amplified differential component
between input signals, while the latter is that of suppression of the
common-mode input signal.
If the ratio of the former value to the
latter one is defined, it can be used as an index for expressing the
quality of differential amplifier since it inevitably indicates the rate
of the differential-mode signal that can be taken out without influ-
enced by the common-mode signal.
The CMRR
is defined as follows:
Vout
+ Vout2
Common-mode
gain (CMG) = {( Sint
eine
CMRR =
—
CMG
Vin1 = Vin2
+ Voce
Output 2
Output
|
7
~- Voc
,
< Differential Amplifier >
DIFFERENTIAL
AMPLIFIER
tn differential amplifier, if Q1 and Q2 are completely
identical with each other, the output being proportional to
difference of input level is generated at the output terminal.
When the same potential in the in-phase is applied to input
1 and input 2 respectively, no output is generated between
output 1 and output 2. This is because the emitter resistor
Re acts as a large negative feedback
for both transistors.
In other words, the output does not come
outsince both
inputs mutually function to cancel the
collecta
currents.
the collector currents.
When
the inputs are mutually
in the antip fhase, the
signal component does not flow through the emitter resist-
ance and the negative feed back disappears.
Thus the emit-
ter resistance does not permit the flow of AC cosnponent.
Since
there is no
feedback,
the gain is increaed
by the
amount
of feedback reduction.
In this manne,
when Re
increases, its self-bias characteristic is emphasiacd and the
negative feedback
is increased.
This state is equ avalent to
the fact that the emitter resistance Re is zero in terms of
AC component.
If this resistance is increased, thi common.
mode component can be distinguished from the d €ferential-
mode component
more effectively, thus oObtairjmg a large
CMRR value.
|
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