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There is a tutorial on multi-stage amplifiers. It doesn't explain every details. The first stage is the typical amplifier with stable bias, and the last stage is the push-pull amplifier. But there is also Q2 which is direct coupled in between. But what is the function of Q2 PNP transistor in the amplifier circuit below? Is that there just to invert the signal?:

enter image description here

UPDATE!: A better version is below. Any one can explain Q2?

enter image description here

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – W5VO Jan 22 '16 at 3:50
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enter image description hereTo User16307: Yesterday I already have tried to answer your question (role of Q2 and why pnp?), however some forum members heavily disagreed. I understand that you feel confused now - and, therefore, I think it is best for you to draw a picture of your own. For this purpose, in the following I have listed some references, which may be helpful:

1) R.C. Jaeger, Microelectronic Circuit Design: "Alternating npn with pnp transistors from stage-to-stage is common in dc-coupled designs"

2) Sedra and Smith, Fig. 7.43, 7.44, 7.49 („..the pnp transistor provides the essential function of shifting the dc level..“.)

3) Univ. of Berkeley: (pages 5...7 ): „By using complementary devices, active level shifting can be combined with amplification“.

http://www-inst.eecs.berkeley.edu/~ee105/fa14/lectures/Lecture22-Multistage%20Amplifiers%282%29.pdf

4) https://wiki.analog.com/university/courses/electronics/text/chapter-10:

Fig. 10.1.4 („By using complementary devices, active level shifting can be combined with amplification“)

5) Carleton Univ, Calif.: Fig. 12-11 http://www.doe.carleton.ca/~rmason/Teaching/486-a.pdf

6) Mass. Inst. of Tech.

http://ocw.mit.edu/resources/res-6-010-electronic-feedback-systems-spring-2013/textbook/MITRES_6-010S13_chap07.pdf Fig. 7-21 (identical to Roberge: Operational Amplifiers, p.280)

7) Ferranti Semiconductor (March 1974), (picture given at the top).

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The other answers are so incorrect.Q2 is what is called VAS, or transimpedance stage; this is an age old idea used in many audio amplifiers today. The purpose is to convert current to voltage; Q1 converts voltage to current, but audio amps are voltage amplifiers, so current has to be convert back to voltage, and then buffered. Q1 also inverts the phase and Q2 puts it back to normal.

First schematics is wrong, because it has too much of a gain (10000 easily), without negative feedback, that drops the gain to the normal values , <100.

EDIT: I thought a bit, and kinda understand where LvW is coming from. First of all yes, you can consider it as a DC shifter (coupled with R2), as it moves Vcc at the collector down to ground level and amplifies, with inversion. But that is not all. Anyway, his explanation is not as wrong as it is incomplete and it uses little too academical language. Besides OP changed the question in a subtle which caused a lot of confusion. I apologize if if was too harsh.

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  • \$\begingroup\$ i also agree that the answers are too academic for me, i just needed such way of explanation: "When the Q1 collector voltage increases Q2 base voltage increases and then so forth and so on..." more easier for a non-expert. that way i can have and idea how and why Q2 is used. the jargon and the level is beyond my level. \$\endgroup\$ – user16307 Jan 21 '16 at 17:22
  • \$\begingroup\$ @W5VO, If you really think this needs to be moved to chat, then so does the question's comment section and my own answer's comment section as well. \$\endgroup\$ – Dave Jan 22 '16 at 0:45
  • \$\begingroup\$ @Dave Done. Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – W5VO Jan 22 '16 at 4:10
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Q2 has a large voltage gain, which is needed for increasing the small swing at Q1 collector up to the rail to rail swing needed for the output push-pull pair, that provide only current gain, but no voltage gain.

Q2 is a PNP because Q1 is NPN. The collector output of Q1 is at the +ve rail, so that's where the next transistor has to have its input. A capacitor cannot be used to couple the signal, because it needs to amplify DC to get the bias conditions right, not just the AC of the signal.

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Q2 both amplifies and DC shifts the voltage present on the output of Q1. This is a very standard technique used to DC couple the transistors and allow for feedback control of the circuit to be added. Often, (basically every op amp I've ever seen and the one I designed in college included) amplifiers have P-type inputs followed by N-type gain stages. There are, of course, more complex designs, but I'm not going over that here.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – W5VO Jan 22 '16 at 3:55
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It is the main purpose of Q2 to bring the DC voltages of the following stage(s) down to lower values. As an counter example, think of three DC coupled npn transistor stages. Each collector node is directly coupled to the base node of the following transistor. This will result in DC potentials (bias conditions) which increase from stage to stage - and will require a corresponding large supply voltage.

In the shown circuit, the collector potential of the pnp transistor Q2 is even LOWER than the collector potential of Q1. As a consequence, the supply voltage can be within suitable limits.

EDIT: The basic question was as follows: "But what is the function of Q2 PNP transistor in the amplifier circuit below? Is that there just to invert the signal?"

As a summary, my answer was that the main purpose of choosing Q2 as pnp is DC level shifting (and obviously gain). But, of course, this does not mean that the whole circuit as shown is the best of all alternatives. But that was not the core of the question. Certainly, the temperature sensitivity of Q2 can be improved using emitter degeneration (as shown, for example, in Sedra/Smith Fig. 7.43 and 7.44).

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – W5VO Jan 22 '16 at 3:56

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