Simple BJT DC Bias Circuits
© Bob York
BJT DC Biasing
+10 V
BJT current gain:
Rc
Design goal:
Ic
Rb
50mA
40mA
400 μA
30mA
300 μA
20mA
200 μA
100 μA
10mA
Vce
Vb
Vout  6 V
I c  20 mA
Vout
Ib
  100
Ic
0 μA
To get a 4V drop across the collector
resistor @ 20mA requires a resistor value
0.7 V
Rc 
10 V  6 V
 200 
20mA
Ib 
Ib
Biasing circuits and load-lines similar to FET circuits.
Consider the design of the circuit to set a certain Q-point:
6
Ic

10
Vce
 200  A
+10 V
Now find the base resistance Rb
that will give this current (KVL):
Vb  I b Rb  0.7 V  0
Rb 
© Bob York
Vb  0.7 V
Ib
If we use the same supply voltage
for Vb (+10V) then
Rb 
10  0.7
 47 k
200  A
200 Ω
47 kΩ
+6 V
  100
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Base-Current Bias Network
In some BJT resistive bias circuits it may help to make use of
Thevenin equivalent circuits for the base-current bias network:
+Vcc
+Vcc
Rth  Rb1  Rb 2
Rc
Rb1
Ic
Vout
Vth  Vcc
Rb 2
Rb1  Rb 2
Ib
Rth
Rc
Ic
Ib
Vth
Rb2
Replace this side by
Thevenin equivalent
Vout
Vout  Vcc  I c Rc
I c   Ib
Vth  I b Rth  0.7 V  0
Then currents/voltages can be found easily as in previous slide
As with FET and diode bias circuits, always check that your final answers are
consistent with any assumptions. For the forward active region:
© Bob York
BE diode forward biased:
vbe  0.6-0.7 V
BC diode reverse biased:
vcb  0.4 V
Vce >0.2-0.3V
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PNP Biasing Examples
Find Ic and Vout
+10 V
0.7 V
Ib
  100
Rb
33 kΩ
Vout
Ic
Rc
150 Ω
Ib 
10  0.7
 280μA
33k
I c   I b  28mA
Vout  I c Rc  4.2V
Vec  10  4.2  5.8V
What would happen if Rc
were increased to 1kΩ?
Vout  I c Rc  28V ?????
This is larger than the supply
voltage, so something has to give.
In this case, Vec would saturate at
around Vec,sat (0.2V) and hence
Vout  10  Vec, sat  9.8V
+10 V
What’s wrong here?
EB diode not forward biased, so
Ic
Rc
© Bob York
Vout
Ic  0
Vout  0
150 Ω
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Stabilizing the Bias Point: Emitter Resistance
Like FETs, the BJT device parameters can vary from device to
device. Another concern is the positive temperature-coefficient of β.
Ic
When current is passed through a device its temperature will
increase. In a BJT, a temperature rise increases the current gain,
allowing more current to flow. This in turn makes the temperature
go up further. In some cases this process may continue until the
device is destroyed. This is called “thermal runaway”.
Adding a resistor in the emitter adds negative feedback to stabilize
the bias point, in the same way that adding a source-resistance
helped in the FET bias circuits.
Rb
If the collector
current
increases for
some reason,
an emitter
resistor will
cause Ve to
rise and thus
lower Vbe
which in turn
reduces Ib and
hence Ic
Ib
Vce
Vb
0.7 V
Re
Ve
Ie
Rb,in
KVL:
I e  (   1) I b
Vb  I b Rb  0.7  I e Re  0
Ib 
Vb  0.7
V  0.7
 b
Rb  (   1) Re Rb  Rb,in
Ic   Ib 
 (Vb  0.7)
Rb  (   1) Re
Note: from base side, Re appears (β+1)
times bigger due to transistor action
For large β the collector current
approaches a value that is
independent of the current gain:
Since β is often large, assuming Ib=0 (infinite β) often gives a
reasonably accurate estimate of the emitter voltage
© Bob York
Ic 
(Vb  0.7)
Re
Ve  Vb  0.7
Rb,in  (   1) Re
as
 
Ie  Ic
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4-Resistor Bias Network
As with FETs, a common bias network for discrete designs
+Vcc
Design Procedure for a given BJT
• Specify Q-point (Ic and Vce)
Ic
Rb1
Rc
Ib
Vce
Ve
Rb2
Ie
Re
slope
• Net resistance can be
determined from load-line:
Vc
Vb
Vcc
Rc  Re
Load-line now a function
of both Rc and Re

1
Rc  Re
Increasing Ib
Ic
Vcc  I c Rc  Vce  I e Re
Rc  Re 
Vcc  Vce
Ic
Vcc
Vce
• Often there will be constraints on Rc or Re imposed by
the application. If Vc or Ve is specified, then
Rc 
Vcc  Vc
Ic
or
Re 
Ve
Ie
• Once Rc and Re are known we can design the base bias network such that:
Vb  Ve  0.7
Ib  Ic / 
• Usually one of the base resistors (Rb1 or Rb2) is fixed at some convenient value
and the other value is then computed to establish the correct base current.
© Bob York
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Diode-Connected BJT
2N3904
25
Vc
Current I, mA
Shorting the collector
and base makes the
device functionally
equivalent to a diode
Ic
Diode-connected BJTs
often have ideality
factors close to 1
20
Data
15
10
5
0
0.4
0.5
0.6
0.7
Diode Voltage V, Volts
Measured Data for a 2N3904 (at right)
T  290 K
I s  20fA (!)
100
n  1.06
Useful in bias circuits as a constant
voltage drop like a Zener diode.
Data
10
Current I, mA
Temperature-dependence is similar
to an ordinary diode
0.8
Model
1
3.642E+01x
y = 2.047E-11e
0.1
0.01
0.001
0.0001
0
0.2
0.4
0.6
0.8
1
Diode Voltage V, Volts
© Bob York
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