State University of New York at Stony Brook
Department of Electrical and Computer Engineering
ESE 211 Electronics Laboratory A
Lab 9. Bipolar Junction Transistor (BJT)
1. Objectives
1) The effect of bias on BJT small signal parameters will be analyzed. The bias
ranges for the cut-off, forward active and saturation modes will be identified.
2) Measurements will be carried out to obtain:
1. BJT input and output characteristics.
2. BJT small-signal parameters: differential input resistance rπ;
differential output resistance r0;
transconductance gm.
The experiments will be performed with n-p-n BJT 2N2222A.
2. Introduction
A bipolar junction transistor can control the amount of power taken from a DC source
using much smaller input signal power, i.e. BJT can amplify power of the signal
source. The power spent by the signal source is determined by input impedance Rin
(Figure 1). The output voltage and output power are defined by the BJT and circuit
parameters.
Vout
Rload
Vin
Rin
Vdc
0
Figure 1.
One can recognize common emitter (CE), common base (CB) and common collector
(CC) BJT configurations in circuits depending on which BJT terminal is grounded (i.e.
used as a reference point for the input and output signals). A BJT gain stage can
amplify voltage (CB), current (CC) or both (CE). In this work we will use only the CE
configuration when the input voltage is applied between the base and emitter
terminals, and the output voltage is taken at the collector with respect to the ground
(emitter).
Electron Energy
The forward bias of the BASE-EMITTER junction lowers the energy barrier for
electrons between the base and emitter (Figure 2). Electrons from the EMITTER
diffuse across the BASE and then drift through the depletion region into the
COLLECTOR.
Diffusion
Emitter
+VBE
Base
+VCE
Collector
Figure 2.
The COLECTOR-BASE junction is reverse biased: the positive COLLECTOR accepts
electrons from the BASE, while creating a barrier for holes. The latter carriers can
only diffuse from the BASE to the EMITTER or recombine in the BASE. Both hole
processes compose the BASE current. To make the base current smaller compared to
the emitter current, the BASE width is made very small and the acceptor concentration
(doping) in the BASE is made much smaller than the donor concentration in the
EMITTER.
Thus, with increase of the BASE-EMITTER bias we get small flow of holes from the
BASE into the EMITTER and large flow of electrons from the EMITTER into the
COLLECTOR. It appears that small BASE current creates much larger COLLECTOR
current. The ratio of two currents is the BJT current gain β.
β=
∂I C ∂I E
, or iC = β iB ,
≈
∂I B ∂I B
(1)
where lowercase letters denote amplitudes of AC signals, and capital letters denote DC
values. The current gain is defined approximately by the ratio of the doping
concentrations in the EMITTER and BASE regions.
For amplification of AC signals it is necessary to apply positive DC voltage to the base
to keep the base-emitter junction forward biased for both half-waves of the AC signal.
If the amplitude of the AC signal is larger than the DC bias – then for the negative
half-wave the base-emitter junction will be under reverse bias. The BJT is switched
off and can not conduct the current; this is the BJT cut-off mode. Both DC voltage
source VBB and resistor Rbias provide stable BJT bias current (Figure 3).
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When the collector-base junction is reverse biased and the base-emitter junction is
forward biased, then the BJT is in the forward–active mode and can amplify the small
signal. The input AC signal with amplitude vBE controls the output AC current with
amplitude iC, i.e. BJT is characterized by transconductance:
gm =
∂I C
, or iC =βiB = gm vBE
∂VBE
(2)
The BJT input characteristic is highly nonlinear because the base current IB depends
exponentially on the base-emitter voltage:
⎞
⎛V
I B ∝ exp⎜⎜ BE − 1⎟⎟
⎠
⎝ VT
(3)
The BJT input impedance for AC signal, rπ, is obtained by differentiation of
equation (3)
∂I B
I
1
= B
=
rπ ∂V BE VT
(4)
One can see that rπ decreases with bias current IB.
In the ideal BJT, the collector current IC is independent of the collector voltage VCE, i.e.
the BJT output characteristic IC (VCE) is a horizontal line that shifts up and down in
accordance with changes in base current IB. In a real BJT, IC increases slightly with
VCE. This dependence can be modeled using an output impedance r0 which is
important for accurately determining the gain value.
r0 =
∂VCE V A
=
∂I C
IC
(5)
Here VA is the Early voltage parameter (typically in the range of 100 - 200 V).
Vout
Rbias
VBB
Rload
C1
Q1
DC
Vin
VCC
2N2222
AC
DC
0
Figure 3.
Resistor Rload converts AC current iC into AC output voltage vout. For a given DC
source this resistor limits the maximum current in the collector path. If input current
amplitude is too large1, then the output current IC saturates at the maximum value of
ICmax = VCC/Rload. When the BJT is in this saturation mode it can not control the output
1
IB + iB is larger than VCC/βRload
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current because the output voltage is near zero. In modern BJTs the typical collectoremitter saturation voltage is 0.2 V. In this mode the collector-base junction is slightly
forward-biased due to difference in the base and collector voltages with respect to the
ground (emitter). In the saturation mode the BJT transconductance is small.
In this experiment the BJT input and output characteristics will be measured and the
small-signal parameters will be determined as function of DC bias.
3. Prelab
Simulate in PSPICE the circuit in Figure 3 with the following parameters:
Rbias = 100 kΩ, C1 = 10 µF, vin = 10 mV, 1 kHz. Obtain the following:
1. Family of the input characteristics: IB (VBE) for VCE1 = 0 V and VCE2= +10
V. Perform the simulation with Rload = 0. Present the input characteristics
on the same plot. Obtain the dependence of rπ on IB.
2. Family of the output characteristics: IC (VCE) for IB ≅ 10 µA and again for
IB ≅ 20 µA. Perform the simulation with Rload = 0. The required values of
VBB can be estimated by Ohm’s law. Present the output characteristics on
the same plot. Find the values of IC corresponding to the given IB currents
at VCE = +5 V. Calculate r0 for these collector current values.
3. Assume VCC = + 10V, Rload = 1 kΩ. Obtain the dependence of iC on IB in
the range of IB from 0 to 100 µA in 20 µA steps. Plot the BJT
transconductance as a function of IB. On the plot show the BJT cut-off,
forward-active and saturation regions.
4. Experiment
Assemble the circuit in Figure 3 with the circuit elements listed in the Prelab and
Rload = 1 kΩ. Perform the following measurements:
1. The dependence IB (VBE) with VCC=+10 V.
2. The output characteristics for IB1 ≅ 10 and IB2 ≅ 20 µA.
3. For vin = 10 mV, 1 kHz and VCC = +10V measure the amplitude of the
output current iC as a function of DC bias current IB in the range of IB
values from 0 to 50 µA.
The DC and AC values are to be measured with oscilloscope probes attached to the
BJT base and collector terminals. Verify your results of DC voltage measurements
with the DMM. The DC currents values are to be calculated from the voltage drop
measurements across resistors with known values.
5. Report
The report should include the lab goals, short description of the work, the
experimental and simulated data presented in plots, the data analysis and comparison
followed by conclusions.
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