Laboratory Manual - King Fahd University of Petroleum and Minerals

where R R 1
// R 2
// r and '
in B B 1
RL
RL
// RC1
2. The low frequency pole can be approximated by the pole due to the capacitor
assoiciated with the lowest resistance namely C E :
1
wL
(2)
r
RS
// RB
1
// RB2
CE
( RE1
//
)
1
3. The high frequency pole can be approximated by the pole due to input capacitance
which becomes significantly large because of miller’s capacitance C eq = C '
(1 g R m
)
1 eq in s 1
m L in s
1 L
:
1
1
wH
(3)
'
[ C C ] R // R C g R ( R // R )
'
Where g m RL
represents the voltage gain between the two terminals of C μ1 (i.e. the gain
between the collector and base of Q1). Since R L is connected at the collector of Q1,
changing its value will directly alter both A M and ω H . In order to maintain almost constant
gain and hence constant bandwidth a buffer stage or common-collector (CC) amplifier
can be used to isolate the load from the basic CE amplifier or C μ1 as shown in Figure 2.
Now the input resistance of the CC amplifier (R in2 ) will act as the new load derived by
the collector of Q1. The input resistance of Q2 is slightly dependent on R L and hence
changing R L will lead to small variation in the gain and ω H . In other words, since the CC
amplifier has a relatively small output resistance which can derive various loads while
maintaining almost constant gain. Keeping the gain almost constant will also results in
constant bandwidth. Also, note that since CC amplifier has usually much larger
bandwidth than that of the CE amplifier, the overall bandwidth of the multistage
amplifier will mainly be decided by the poles of CE amplifier.
+
5V
-
R Sig
Vsig
Vcc
1k
C 1
1
R B1
R C1 4.3k
30k
C 2
Q1
1
2N3904
R B2 10k
R E1 1.3k
C E
R B3
R B4
47
30k
10k
4.3k
R E2
2N3904
1
C 3
1.3k 100k R L
V out
Figure 2: Common Emitter amplifier with Buffer
With the buffer connected the midband the various parameters become:
v
o
Rin
''
1. A M
g
mRL
ABuffer
(4)
v R R
sig
sig
in
23