General PNP transistor circuit

Thread Starter

pinkyponky

Joined Nov 28, 2019
6
Hello all,

I have simulated the circuit in LTspice to understand the basic functionality of the PNP transistor circuit and I couldn't understand the behavior and questions are listed below. Please could you answer them.

1. Why the circuit yields the different base voltages that are measured at the Vb1 and Vb2 points are shown in the below circuit?. As per my understanding the base voltage of Vb1 and Vb2 should be same, Right?.
2. What is purpose of resistor R3? and why we need to use resistor R3?


1587141214215.png
 
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Papabravo

Joined Feb 24, 2006
13,732
R3 allows the transistor base to be pulled down to 4.3 Volts (about 0.7 V below the 5V source)
When V1 and V2 are both at 5V there is no current flowing in R3, R1 or through the BE junction of Q1
When V2 goes to GND there is 0.7V across R3 and 4.3 V across R1 and some current through the BE junction of Q1 and the transistor is on
You need R3 because you cannot pull the base of the transistor below 4.3 volts. You would essential be shorting two supplies together across the BE junction
 

dl324

Joined Mar 30, 2015
10,750
Schematics are easier to read when they're not drawn upside down:
1587143875667.png
1. Why the circuit yields the different base voltages that are measured at the Vb1 and Vb2 points are shown in the below circuit?. As per my understanding the base voltage of Vb1 and Vb2 should be same, Right?.
vb1 and vb2 will only be at the same voltage when there is no current in R1.
2. What is purpose of resistor R3? and why we need to use resistor R3?
R3 turns the transistor off when there's no input on vb1.
 

crutschow

Joined Mar 14, 2008
24,969
You need R3 because you cannot pull the base of the transistor below 4.3 volts. You would essential be shorting two supplies together across the BE junction
Not really.
The base current is basically controlled by R1.
That resistor drops the 4.3V between the supply and the base when the input is 0V.

R3 is not absolutely needed at normal room ambient, but it's typically added to shunt away any base leakage current that would tend to turn the transistor on at high temperatures.
It also helps some with speeding up the transistor turn-off.

You will find that the simulation looks essentially the same with or without R3.
 
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Thread Starter

pinkyponky

Joined Nov 28, 2019
6
R3 allows the transistor base to be pulled down to 4.3 Volts (about 0.7 V below the 5V source)
Thank you for your explanation and partially I understand. My another question is on resistor R3, how the resistor R3 pulled down the 5V to 4.3V, could you give some more detailed.

When V2 goes to GND there is 0.7V across R3 and 4.3 V across R1 and some current through the BE junction of Q1 and the transistor is on
I have simulated the circuit again and I measured that there is a 4.3V across R1 and R3, when the V2 goes to GND. I could not measured that 0.7V acroos the R3.

And also please could you explain why we need to use and purpose of the resistor R1 and R2.
 

WBahn

Joined Mar 31, 2012
25,760
If you measured 4.3 V across both R1 and R3, then, depending on polarity of your measurements, you have 8.6 V across the pair. Does that make sense? You made a mistake in your measurement.

You already been told that R1 controls the base current. Without it you will be directly driving the base emitter junction with a voltage source and will let the magic smoke out.

R2 is there so that you can get an output signal. Without it, the voltage across the collector-emitter junction would be fixed and while you might be controlling the current in the transistor, so what? In most (not all) circuits you want to produce a useful signal. The resistor acts as a current to voltage converter.
 

crutschow

Joined Mar 14, 2008
24,969
Below are the node voltages for your circuit when the transistor is on.
The voltage across R3 is V(e) - V(vb2) = .73V.
Also note that the collector voltage V(c), is very near the supply voltage, showing the transistor is fully on and saturated.

Note that R1 is much smaller than it needs to be.
A value 20k for R2 is sufficient to fully turn on the transistor.

1587226219371.png
 
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