Transistor DC voltmeter circuits, Emitter follower DC voltmeter, Op-Amp voltage follower DC Voltmeter, Amplifier based DC voltmeter for low voltage measurement, Op-Amp amplifier DC Voltmeter

1. Electronic DC Voltmeter
Unit II Electronics & Digital meters
2/22/2017 1
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

2. Limitations of PMMC / multiplier Voltmeters
PMMC Voltmeter has poor sensitivity and can not measure very
low voltage
input impedance is quite low for measurement in high impedance
circuit.
Low input impedance may alter the circuit voltage thereby giving
loading effect
To measure low voltages it need to be amplified
These limitations are overcome in Electronic Voltmeter
2/22/2017 2
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

3. Electronic DC Voltmeters
2/22/2017 3
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
high input impedance of amplifier reduce loading effect
low voltage ranges can be measured after amplification
Voltage range selection can be made using input attenuator
PMMC
RS
RCES
Vo
1 mA
IC
-
Amplifier
High input impendence
low output impendence
Attenuator or
Voltage Range
selection
E

4. Transistor voltmeter circuits
Emitter Follower Voltmeter
• It greatly reduces Voltmeter loading
• Emitter follower has high input impedance and low output
impedance (to drive coil of PMMC)
2/22/2017 4
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
PMMC
RE
IB
RC
E VE
VBE
1 mA
IE =IC
VCC
+
-
+
-
RI=E/IB
C
fe
B
i
fe
C
fe
E
B
I
hE
I
E
R
h
I
h
I
I
.
and 


5. Transistor voltmeter circuits
Emitter Follower Voltmeter of previous slide has VCC=30 V, meter circuit
resistance RE+RC= 4.65 K, IC= 2 mA at full scale, hfe= 250. Given VBE=0.7 V
(i) Calculate meter current when E=10V
(ii) Find Voltmeter Input resistance with or without transistor.
2/22/2017 5
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
mA
K
V
RR
V
I
VVEV
CE
E
C
BEE
2
65.4
3.9
3.97.010







 
KRRR
M
I
hE
I
E
R
CEi
C
fe
B
i
65.4ansistorwithout tr
25.1
10x2
250x10.
rtransistowith 3

6. Transistor voltmeter circuits
In Emitter Follower Voltmeter, VBE leads to error, which can be
eliminated by using a Voltage divider and a emitter follower circuit
2/22/2017 6
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
At E=0, Base voltage of Q2 is
adjusted to give IC=0, i.e
VP=0, and VE1=VE2=-0.7 V
For other E
V= VE1-VE2 = (E-VBE1)-VBE2
= E-VBE1+VBE2 = E
Zero Voltage terminal of the circuit
IB1
E
R3
R4
R5
R6
Q1 Q2
R2
R1
I2 I3
VE2
VP
IB2
I4
VCC
VEE
+
-
V
PMMC
RS
IC
RC
VE1

7. Transistor voltmeter circuits Various ranges
Voltage divider circuit (attenuator) allows various voltage ranges to be
measured. Input FET stage further increases input resistance
(typically > 1 M)
2/22/2017 7
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
25 V
10 V
5 V Q3Ra
Rb
Rd
Rc
E
800 K
100 K
40 K
60 K
EG
1 V
Input Attenuator
FET
input
Emitter follower Voltmeter
R3
R4
R5
R6
Q1 Q2
R2
R1
I2 I3
VE2
VP
IB2
I4
VCC
VEE
+
-
V
PMMC
RS
IC
RC
VE1
VGS
IS

8. Transistor voltmeter circuits Various ranges
Ex. In the Multiple range voltmeter, based FET input Emitter
follower voltmeter, determine the meter reading when E=16 V,
meter range is 25 V. FET gate source voltage in -5 V, Vp=5 V,
RS+RC= 2K, and Im = 0.5 mA at full scale.
2/22/2017 8
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
FSD)of%(6432.0
2
64.0
64.03.494.4
3.47.05
94.47.064.5
64.5)5(64.0
64.0
8001006040
40
16Erange,25VOn
21
2
1
G
mA
KRR
V
I
EVVVV
VVVV
VVVV
VVEV
V
RRRR
R
E
CS
C
GEE
BEPE
BESE
GSGS
dcba
d













9. Difference Amplifier Voltmeter
•Transistors based voltmeters can measure from 100 mV to 25 V
•For low voltage measurements, it need to be amplified
•can be done by differential or emitter coupled amplifier voltmeter
•When base voltage of Q2 is zero, difference of two base voltages is
amplified and applied to meter circuit
2/22/2017 9
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Q1
RE
IC1
VCC
VEE
+
-
RL2
Q2
V
PMMC
RS
IC
RC
VC1 VC2
R2R1
IC2
IE1 IE2
E
RL1
R3
•For E=0, R3 can be adjusted
for zero meter reading
•When E is +ve, IC1  & IC2 
•Resulting in VC1  & VC2 
•Thus V= VC2 – VC1  E

10. Op-Amp Voltage follower Voltmeter
It is similar to emitter
follower voltmeter in
working.
It offers high input &
lower output impedance
than emitter follower
Also there is no base
emitter voltage VBE drop
Op-Amp Voltage follower
configuration ensures that
VO=EG
2/22/2017 10
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Input
Attenuator
Ra
Rb
Rd
Rc
E
EG
Voltage follower
Meter
circuit
PMMC
RS
RC
IC
Vo
+
-
RF
+VCC
-VEE

11. Op-Amp Amplifier Voltmeter
Low voltages can be amplified using
Op-Amp amplifiers
It offers very high input & lower output
impedance than emitter follower
2/22/2017 11
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
R
E
Non inverting
amplifier
Meter
circuit
PMMC
RS
RC
IC
Vo
+
-
RF
+VCC
-VEE







R
R
EV F
o 1