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I am having trouble understanding the three operation modes of a transistor. I am giving below the three operation modes (CB, CE and CC) for pnp and npn transistors. enter image description here

and for npn , here it is. enter image description here

Unfortunately , I couldn't get the same format of picture for both types and its strenuous to draw each circuitry in circuitlab.

in CB mode, the EB junction is forward biased and CB junction reverse biased.

in CE mode , the BE junction is F.biased and CE junction is R.biased.

in CC mode , the BC junction is F.biased and EC junction is R.biased.

I am having no trouble understanding the common base configuration for both of them. But I am having trouble understanding the other two. Consider the CE mode. The collector and Emitter both are negative (for npn) , then how can we reverse bias them? does that mean that the collector is reverse biased with respect to base and vice versa ? I want a full detailed description of how the CE and CC mode works .

Question 2 : Why is the common pin (the base , the emitter , and the collector respectively) grounded? what is the significance of grounding them?

Question 3 : How does a CE mode amplify more than a CB mode ?

Thank you.

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  • \$\begingroup\$ That's a lot of questions, and you seem to be confusing yourself with your terminology. In every case, The B-E junction is forward-biased, and the B-C junction is reverse-biased. There is no C-E junction as such, but in every case (for the NPN), the collector is more positive than the emitter. \$\endgroup\$
    – Dave Tweed
    May 26, 2015 at 14:48
  • \$\begingroup\$ That's what I was thinking. Its no such thing as CE junction, it can't be. By connecting the battery in the above formation we are simply changing the bias of CB and EB each time. But unfortunately that's the thing written in my text book. So I got confused. Please tell me if I'm right \$\endgroup\$
    – Shiladitya Bose
    May 26, 2015 at 14:55
  • \$\begingroup\$ There is no signal on the "common" terminal. In the CE circuit, input is on the base, and output is on the collector. For CC, input is on the base, and output is on the emitter. \$\endgroup\$
    – Peter Bennett
    May 26, 2015 at 15:13
  • \$\begingroup\$ Another question, I think that the common collector diagram for pnp on the above picture is wrong. The Base should be connected to -ve and collector to +ve of the left battery and emitter to -ve on the right. Am I right? \$\endgroup\$
    – Shiladitya Bose
    May 26, 2015 at 15:25

2 Answers 2

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I am having trouble understanding the three operation modes of a transistor.

When we talk about the modes of operation of a transistor, we're usually talking about cut-off, forward-active, and saturated operation.

The rest of your question seems to be about the different fundamental amplifier configurations, rather than the operating modes, so that's what I'll answer about.

Consider the CE mode. The collector and Emitter both are negative (for npn)

Check your diagrams again. For an NPN CE stage, the base and collector are both biased at higher potential than the emitter.

then how can we reverse bias them?

For a CE stage, the base-emitter junction should be forward biased; the base-collector junction is reverse biased. This is achieved by biasing the collector at a higher potential than the base. This is exactly what's shown in the diagram you posted.

Why is the common pin (the base , the emitter , and the collector respectively) grounded? what is the significance of grounding them?

They aren't necessarily grounded. They are connected to some potential that is equivalent to ground in the ac equivalent circuit. Particularly for common-base stages or PNP common-emitter stages, this is usually not the same as the circuit ground.

The significance is that this node is "common". A node that is used in common between the other nodes to define their potentials. The fact that the emitter is connected to the common node is why we call a common-emitter stage a common emitter stage.

How does a CE mode amplify more than a CB mode ?

A common emitter stage has voltage and current gain. A common base stage is essentially a unity gain current buffer. You need to study the common emitter stage to understand why it has voltage and current gain, and study the common base stage to understand why it is a unity gain buffer. Once you understand those two things, you'll understand why the one stage has more gain than the other.

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All the time think of more positive thisway: it does not mean positive in the sense of polarity. Vb= 5v and vc= 3v are both positive voltages however voltage at point b is more positive than point c and if you place a diode where the anode connected to b @ 5v and the cathode connected to c @ 3v the diode will be forward biased connecting the diode anode to point c @ 3v and the cathode to point b @ 5v The diode will be reverse biased. The same way if vb= -4v and vc= -6 @ point b the voltage is more positive Than the voltage @ point c because -4 > -6 and ( -4)-(-6)= +2 So placing a diode such the anode is connected to (-4v) and the cathode to (-6v) we have a forward biased diode the opposite will be rev bias.

Vb =0 v & vc = -2v anode at b and cathode at c -2v will give as a forward biased diode et vise versa.

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