This document discusses various types of electronic voltmeters. It includes figures and explanations of transistor voltmeters using emitter followers, differential amplifiers, and operational amplifiers. Key aspects summarized include: - Figure 4.1 shows a transistor voltmeter using an emitter follower where the input voltage is stepped down by the transistor's VBE drop and the input resistance is increased. - Figures 4.2 and 4.3 demonstrate how the difference between two emitter voltages VE1 and VE2 equals the measured voltage Vm for different input voltages Vin in a differential amplifier configuration. - Figure 4.4 presents a circuit that uses a differential amplifier to step up a small input voltage to 75% of the

1. ELECTRONIC INSTRUMENTS

3. Electronic voltmeter

4. FIGURE 4.1
Transistor Voltmeter;Emitter Follower

5. Question
PBL : ELECTRONIC INSTRUMENTATION
FIGURE 4.1
• VE=Vin-VBE=10 V-0.7 V=9.3V
• Im=VE/(RS+Rm)≅9.3 V/9.3k Ω=1mA
• With transistor
• IB ≅Im/hfe=1mA/100=10 µA
• Ri ≅Vin/IB ≅10V/10 µA=1MΩ
• Without transistor= Ri=Rs+Rm=9.3 kΩ

6. FIGURE 4.2

7. Assignment (14/Sept)
• Derive VE1-VE2=Vm. Draw the equivalent
circuit.

9. Solution
When Vin=1 V
VE1=Vin -VBE =1 V -0.7 V =0.3 V
VE2
=Vp - VBE =0 V – 0.7 V =-0.7 V
Vm=VE1-VE2 = 0.3-(-0.7)=1V
When Vin=0.5 V
VE1=Vin -VBE =0.5 V -0.7 V =-0.2 V
VE2
=Vp - VBE =0 V – 0.7 V =-0.7 V
Vm=VE1-VE2 = -0.2-(-0.7)=0.5V

10. When Vin=7 V
VE1=Vin -VBE =7 V -0.7 V =6.3 V
VE2
=Vp - VBE =0 V – 0.7 V =-0.7 V
Vm=VE1-VE2 = 6.3-(-0.7)=7V

12. FIGURE 4.4
R1

13. EG=E x (Rc+Rd)/(Ra+Rb+Rc+Rd)
=7.5 V x (60k+40k) / (800k +100k +60k +40k)
= 0.75 V
Vs = EG-VGS=0.75 V –(-5 V) =5.75 V
VE1=Vs – VBE=5.75 V – 0.7 V=5.05 V
VE2=VP – VBE=5 V – 0.7 V =4.3 V
V = VE1 – VE2 =5.05 V – 4.3 V
=0.75 V = EG
Im = V / RS + Rm =0.75 V/1kohms
=0.75 mA
75% of full scale

14. Op-Amp Voltage-Follower Voltmeter
Input attenuator

15. FIGURE 4.7

16. Vout = Im x Rm =100µA x 10kΩ = 1V
R3 = E/ I4 =20mV/ 0.2 mA =100Ω
R4 = (Vo – E) / I4 = (1 V -20 mV) / 0.2 mA
=4.9 kΩ

17. FIGURE 4.8

18. The small voltage drop across
shunt resistor Rs is amplified
before being applied to
deflection instrument.
This approach is just as applicable to
the measurement of low level alternating
Currents as it is to direct-current measurement
FIGURE 4.14

19. FIGURE 4.9

21. FIGURE 4.10