Series NPN and PNP transistors

Thread Starter

pinkyponky

Joined Nov 28, 2019
118
Hi all,

Please can you:
1) Explain the functionality of series NPN and PNP transistors?
2) What is purpose of these series of transistors?
3) Please can you explain funtionlity with current and voltage waveforms?
 

Papabravo

Joined Feb 24, 2006
16,772
Hi all,

Please can you:
1) Explain the functionality of series NPN and PNP transistors?
2) What is purpose of these series of transistors?
3) Please can you explain funtionlity with current and voltage waveforms?
1) NPN and PNP transistors are semiconductor devices that can provide current gain. That is, you put a small current into a device, and, using a handy power supply, it makes an exact replica of the input waveform but with a much higher current. A device which provides current gain can also provide voltage gain and power gain, again with the hep of a handy power supply. I hope you didn't think that amplification amounts to getting something for nothing. That can't happen.
2) The main reason for the two types is that hey were conceived and fabricated independently of each other. Practically speaking you can build the same type of circuits with either type, but you power supply voltages will be different.
3) If you have an example circuit I can generate current and voltage waveforms. In the meantime here is a sample. It shows a typical common emitter class A amplifier, with the magnitude and phase response as a function of frequency for the selected transistor.
 

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Thread Starter

pinkyponky

Joined Nov 28, 2019
118
No, we cannot at the moment until we see your circuit diagram.

Hi MrChip,

Yes, I knew that NPN transistor will turn ON when the base voltage 0.6V higher than the emitter voltage. Similarly, the PNP transistors will turn ON, when the base voltage is 0.6V lower than the emitter voltage.

This principle I knew it, when the emitter is connected to the ground through resistor where the NPN transitor is used in the circuit and when the collector is connected to the ground through resistor where the PNP transistor is used in the circuit.

Here, I have a question on the series NPN and PNP transitors.

In series NPN and PNP transistors, Both base are tied together and connected to the voltage source and both emitters of the NPN and PNP transitor is also tied together. I have questions from the below circuit. So, in this case:
1) Is the both transistors will be turn ON at the same time, since the base voltage applied to the transistors are at the same time?.

2) According to the working principle of the PNP transistor (which is explained above), the transistor will turn ON when the base voltage is lower (0.6V) than emitter voltage. But here, the base voltage is higher than the emitter voltage. So, meaning that, the PNP transistor will not be turn ON. If so, why need be used PNP transitor series with the NPN transistor?.

3) What is the main purpose of the series NPN and PNP transistors?. In which applications the circuit will be used?.

I think now you had a nice points to help me, rather than nothing.
 
Last edited:

ericgibbs

Joined Jan 29, 2010
13,579

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LvW

Joined Jun 13, 2013
1,299
Yes, I knew that NPN transistor will turn ON when the base voltage 0.6V higher than the emitter voltage. Similarly, the PNP transistors will turn ON, when the base voltage is 0.6V lower than the emitter voltage.
You are correct when you state that it is the voltage between base and emitter that "turns on" the transistors .
However, in case you want a linear operation (as for amplifiers) it is better (even more correct) not to speak about "turn-on".
The collector current Ic is a function of the base-emitter voltage Vbe with an exponential characteristic - that means, there is a (rather small) current Ic also below this 0.6 volts.
Now, turning to your question:
The combination of npn and pnp transistors is mostly used with respect to the DC quiescent voltages.
If you want to use two DC-coupled transistors in series (example: npn-differential amplifier with an additional stage), the DC quiescent voltage would increase from stage to stage:
* First stage: Diff. amplifier (T1 and T2) with DC voltages Vc1,2 at the collectors (some volts).
* Next stage (T3): Vb3=Vc1,2 (DC coupling) and Vc3 some volts above Vc1,2.

This stepwise DC increase is not wanted - it can be avoided by using a pnp instead of npn. In this case, the emitter voltage of T3 is only 0.6V above Vb3 and the collector voltage Vc3 is even below Vc1,2.
 

Papabravo

Joined Feb 24, 2006
16,772
Hi MrChip,

Yes, I knew that NPN transistor will turn ON when the base voltage 0.6V higher than the emitter voltage. Similarly, the PNP transistors will turn ON, when the base voltage is 0.6V lower than the emitter voltage.

This principle I knew it, when the emitter is connected to the ground through resistor where the NPN transitor is used in the circuit and when the collector is connected to the ground through resistor where the PNP transistor is used in the circuit.

Here, I have a question on the series NPN and PNP transitors.

In series NPN and PNP transistors, Both base are tied together and connected to the voltage source and both emitters of the NPN and PNP transitor is also tied together. I have questions from the below circuit. So, in this case:
1) Is the both transistors will be turn ON at the same time, since the base voltage applied to the transistors are at the same time?.

2) According to the working principle of the PNP transistor (which is explained above), the transistor will turn ON when the base voltage is lower (0.6V) than emitter voltage. But here, the base voltage is higher than the emitter voltage. So, meaning that, the PNP transistor will not be turn ON. If so, why need be used PNP transitor series with the NPN transistor?.

3) What is the main purpose of the series NPN and PNP transistors?. In which applications the circuit will be used?.

I think now you had a nice points to help me, rather than nothing.
Sorry, I misunderstood the use of series as referring to a class of parts rather than as a circuit with two devices connected together. This is more like what you are talking about. The benefit of this kind of output is that it actively pulls the output close to the supply rails and can source or sink current to charge and discharge a MOSFET gate or a speaker, represented by the large capacitor and the 16 Ω resistor.
 

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MrChips

Joined Oct 2, 2009
24,134
Yes, I knew that NPN transistor will turn ON when the base voltage 0.6V higher than the emitter voltage. Similarly, the PNP transistors will turn ON, when the base voltage is 0.6V lower than the emitter voltage.
There is another (better?) way of stating this.

On an NPN transistor, the transistor begins to conduct when the base-emitter voltage exceeds 0.6V
On a PNP transistor, the transistor begins to conduct when the emitter-base voltage exceeds 0.6V.
The difference here is the polarity of the voltage difference.

1627310225382.png
 

atferrari

Joined Jan 6, 2004
4,424
Hi all,

Please can you:
1) Explain the functionality of series NPN and PNP transistors?
2) What is purpose of these series of transistors?
3) Please can you explain funtionlity with current and voltage waveforms?
Not a reply to your questions but could be of interest.

In this thread I asked about a bipolar PSU.

I finally implemented the circuit below for a quick test of several small motors.

Simple bipolar PSU.png
 
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