Transistor Circuits XIII
Common-Collector circuits:
Two-supply biased
Two-supply biased common-collector
circuit
VCC
C1
Output
vs
RB
RE
vo
VEE
AC equivalent
Output
vs
RB
RE
vo
Rearranged ac equivalent
RE
vs
RB
vo
Two-supply biased common-collector “working”
into a useful load equivalent (RL)
VCC
C1
C2
vs
RB
RE
RL
VEE
vo
Signal “sees” RE and RL in parallel
vs
RB
RE
RL
vo
AC load rL is resistance of RE and RL in parallel
rL
vs
RB
vo
Formulas
• 𝐼𝐸 ≅
𝑉𝐸𝐸 −𝑉𝐵𝐸
𝑅
𝑅𝐸 + 𝐵 ℎ
𝐹𝐸
If hFE is large, RB is not too large (< 4.7-kΩ) and
VBE = 0 V, we can simplify this to:
• 𝐼𝐸 ≅
𝑉𝐸𝐸
𝑅𝐸
Normally working circuit: VRE = VEE, VE = 0 V; this
makes VCE:
• 𝑉𝐶𝐸 ≅ 𝑉𝐶𝐶
• 𝐼𝐸(sat) =
𝑉𝐶𝐶 +𝑉𝐸𝐸
𝑅𝐸
• 𝑉𝐶𝐸(cutoff) = 𝑉𝐶𝐶 + 𝑉𝐸𝐸
• 𝑟𝐿 =
𝑅𝐸 𝑅𝐿
𝑅𝐸 +𝑅𝐿
First example circuit
15V
vs
33kΩ
1.2kΩ
-15V
800Ω
vo
Referring to the circuit
shown, find the dc emitter
and collector currents,
collector-to-emitter voltage,
and collector-to-ground
voltage. Determine its IE(sat)
and VCE(cutoff). Assume that
the base-to-emitter voltage
is 0.6 V and that hfe = hFE =
120.
Work for first example
• 𝐼𝐸 ≅
𝑉𝐸𝐸 −𝑉𝐵𝐸
𝑅𝐸
14.4
1.475kΩ
𝑅
+ 𝐵
=
ℎ𝐹𝐸
15−0.6
1.2kΩ+33kΩ 120
=
14.4
1.2kΩ+275Ω
= 9.763mA ∴ 𝐼𝐶 = 9.763mA
• 𝑉𝐶𝐸 ≅ 𝑉𝐶𝐶 = 15V
• 𝑉𝐶 = 𝑉𝐶𝐸 = 15V (Since 𝑉𝐸 ≅ 0V)
• 𝐼𝐸(sat) =
𝑉𝐶𝐶 +𝑉𝐸𝐸
𝑅𝐸
=
15+15
1.2kΩ
=
30V
1.2kΩ
= 25mA
• 𝑉𝐶𝐸(cutoff) = 𝑉𝐶𝐶 + 𝑉𝐸𝐸 = 15 + 15 = 30V
=
Second example circuit
12V
750Ω
50Ω
vs
15kΩ
-12V
vo
• In the circuit shown,
solve for the dc emitter
and collector currents,
the collector-to-emitter
voltage, the collectorto-ground voltage, and
find its IE(sat) and
VCE(cutoff). Assume that
VBE = 0.5 V and hfe = hFE
= 80.
Work for second example
•
𝑉𝐸𝐸 −𝑉𝐵𝐸
12−0.5
𝐼𝐸 ≅
=
=
𝑅𝐵
15kΩ
750Ω+
𝑅𝐸 +
80
ℎ𝐹𝐸
11.5
11.5V
=
= 12.267mA
750Ω+187.5Ω
937.5Ω
∴ 𝐼𝐶 =
12.267mA
• 𝑉𝐶𝐸 ≅ 𝑉𝐶𝐶 = 12V
• 𝑉𝐶 = 𝑉𝐶𝐸 = 12V (Since 𝑉𝐸 ≅ 0V)
• 𝐼𝐸(sat) =
𝑉𝐶𝐶 +𝑉𝐸𝐸
𝑅𝐸
=
12+12
750Ω
=
24V
750Ω
= 32mA
• 𝑉𝐶𝐸(cutoff) = 𝑉𝐶𝐶 + 𝑉𝐸𝐸 = 12 + 12 = 24V
Third example circuit
15V
vs
33kΩ
1.2kΩ
-15V
800Ω
vo
• If the transistor
becomes defective (acts
like an open between
its collector and emitter
leads) in the circuit of
the first example, what
dc voltages can we
expect to measure at
the collector and at the
emitter with respect to
ground?
Work for third example
• 𝑉𝐶 = 15V
• 𝑉𝐸 = −15V
Fourth circuit example
12V
750Ω
50Ω
vs
15kΩ
-12V
• Referring to the problem
from the second example,
if the dc voltage at the
emitter with respect to
ground measures -12 V,
what are the possible
vo
causes of this symptom?
Can the circuit amplify
signals without
distortion? Why or why
not?
Work for fourth example
• Shorted transistor.
• No!
• No transistor means nothing to do work.
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