This document describes the design and testing of a circuit to measure the beta (β) value of NPN bipolar junction transistors (BJTs). The circuit uses a constant current source to pass a fixed current through the collector-emitter path of the BJT being tested. The voltage across a resistor in this path corresponds to the β value in millivolts, which can be read on a digital voltmeter. The constant current source and full β-meter circuit are described in detail. Testing showed close agreement between β values measured by the circuit and those from a digital multimeter.

1. EC 204: ANLOG CIRCUITS LABORATORY
Experiment 1
β‐meter
Manzil Zaheer 08010252
Payoj Kissan 08010823
Sachin Rajoria 08010826
To design a circuit that measures β of NPN BJTs by outputting a voltage equal to “β” mV that can be
displayed on a DVM.

2. Objective:
To design a circuit that measures β of NPN BJTs by outputting a voltage equal to “β” mV that can be
displayed on a DVM.
Instruments/Materials Required:
1. Battery of Voltage: 7.5V x 1
2. Transistors:
i. PNP transistor CK100 x 1
ii. NPN transistor to be checked x 1
3. Zener diode
i. 2.7 V x 1
ii. 4.7 V x 1
4. Resistors:
i. 100 Ω x 4
ii. 200 kΩ x 2
iii. 330 Ω x 1
5. Digital voltmeter x 1
6. Breadboard x 1
Design Methodology:
The idea is to measure voltage across a suitable resistor which will represent the β value in millivolts.
Main part of β‐Meter
We know that for a BJT,
If IB is kept constant and voltage (Vo) across RC is observed as shown in
Fig. 1, then . By choosing proper values of IB and RC we can
obtain Vo = “β” mV, so that when Vo is displayed on a digital voltmeter
(DVM) in mV range, it indicates the value of β.
But for choosing the value of IB and RC, we need to take care of the
power dissipation capacity of the components. Specifically the
resistors can only dissipate (in our case 250 mW). For the worst
case that emitter and collector terminal of the BJT are shorted for
some reason, our device should not get damaged. So the minimum RC,
is given by
Figure 1: Idea of β‐meter

3. Further from this we can get,
For displaying Vo = “β” mV we need that 1 mV,
10 10
A constant current source is required to make IB immune to disparity in VBE of the transistors being
tested. Plus the constant current source should be able to drive a grounded load. We used the following
simple transistor based current source for this purpose.
The Constant Current Source
The Fig. 2 shows proposed constant current source. DZ is a zener diode which, when reverse biased (as
shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it.
Thus, as long as the zener current (IZ) is above a certain level (IZ‐min, called holding current), the voltage
across the zener diode (VZ) will be constant. Resistor R1 supplies the zener current and the base current
(I’B) of PNP transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter
resistor R2. The operation of the circuit is as follows:
Voltage across R2 (VR2) is given by VZ + VBE, where VBE is the base‐emitter drop of Q1. The emitter current
of Q1 which is also the current through R2 is given by
Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is
constant and hence I’E is also constant. Due to transistor action, emitter current I’E is very nearly equal to
the collector current I’C of the transistor (which in turn, is the current through the load). Thus, the load
current is constant (neglecting the output resistance of the transistor due to the Early effect) and the
circuit operates as a constant current source. As long as the temperature remains constant (or doesn't
vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2
allows the load current to be set at any desirable value and is calculated by
The resistor R1 has to be chosen keeping in mind the holding current of zener diode,

4.
Voltage Regulation of CCS: This circuit design is immune to voltage variation in VCC. Let the variation in
VCC be ∆ , then the voltage at base of the transistor Q1 is ∆ , and hence at the
emitter is ∆ . Thus the voltage across the resistor R2 is
∆
∆ ∆
Hence the variation in the source voltage does not affect the voltage across VR2 and in turn the constant
current output,
Overall Voltage Regulation
To obtain further voltage stability, we add a zener diode
parallel to the whole circuit, as shown in Fig. 3. This makes
the circuit further independent of the voltage, provided it is
higher than the breakdown voltage of the zener diode.
Figure 2: Design of Constant Current Source
Figure 3: Use of zener
as voltage stabilizer

5. Calculations:
Let us choose the VBatt to be 7.5V (a standard value). Then, the other circuit parameters are as follows:
 Effective Supply Voltage (VCC) [Across the zener diode of 4.7 V]
4.7V
 Collector Resistance (RC)
4.7
0.25
88.36 Ω
Choose 200 Ω
 Constant Current (IB) from CCS
10 10
200
5 10 A 5 A
 Transistor (Q1) in the CCS: PNP transistor CK100 with
0.7 V
(Assumed, datasheet not available)
 Zener Diode in CCS
2.7 V, 5 mA
…….
 Resistor (R2) in CCS
2.7 0.7
5 10
4 10 Ω 400 kΩ
 Resistor (R1) in CCS
4.7 2.7
5 10
400 Ω
Choose 330 Ω
(To allow slight variations in supply voltage)

6. Circuit Diagram
Figure 4: The complete β‐meter circuit
VCC

7. Observations:
1. Testing of the constant current source:
Sr. No. VBatt RLoad IB (by CSS)
1. 5.5 V ‐ 5 µA
2. 5.5 V 100 Ω 5 µA
3. 7.5 V ‐ 5 µA
4. 7.5 V 100 Ω 5.1 µA
5. 11.5 V ‐ 5.8 µA
2. β measured by the above designed circuit:
Sr. No. Transistor β (DMM) β (Circuit)
1. Q1 (2N2222) 161 153
2. Q2 (2N2222) 169 168
Result:
1. The β‐meter was successfully constructed using discrete components and demonstrated with
immunity to slight variations in the supply voltage.
2. β of the given transistor was found to be quite close to the true value by the use of above
designed circuit.
i. β : 153 by the circuit as compared to 161 by DMM
ii. β : 168 by the circuit as compared to 169 by DMM

8. Comments:
The circuit’s reliability can be improved by introducing a diode made of same material of the PNP
transistor in CCS.
Temperature changes will change the output current delivered by the circuit of Fig. 2 because VBE is
sensitive to temperature. Temperature dependence can be compensated using the circuit of Fig. 5 that
includes a standard diode D (of the same semiconductor material as the transistor) in series with the
Zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to
temperature and thus significantly counteracts temperature dependence of the constant current
source.
Resistance R2 is now calculated as,
Since ,
(In practice VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than
nulling it out.)
Figure 5: Proposed improvement in β‐meter circuit