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What is the Q-point of a transistor?

Why it is needed?

How can we set this Q-point of transistor?

Please explain it as I am having difficulty understanding this concept.

Chetan Bhargava
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Ali Khan
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    "Q point" stands for quiescient point, which is merely the DC operating point of the circuit. I go into some length describing how a transistor is biased, which is the same as determining its Q point, in this answer: http://electronics.stackexchange.com/a/60711/4512 – Olin Lathrop Mar 19 '13 at 13:33

3 Answers3

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The quiescent (quiet) point are the values of the voltages and currents of a circuit when no signal is present.

From Wiki:

The operating point of a device, also known as bias point, quiescent point, or Q-point, is the steady-state voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied.

The choice of Q-point depends on the application and is a crucial part of the design process. The desired Q-point is an input to the DC bias design equations for the circuit.

Alfred Centauri
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  • Perhaps for those who already know what is the Q point, know that when you say “signal” you mean the input (usually sinusoidal) signal. But actually, when a transistor or diode is biased, a DC signal is applied. That is, a DC signal is still applied when a transistor/diode has a Q point. Just a clarification. :) – alejnavab Dec 11 '22 at 18:16
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Q means quiescent operating point and in simple terms it is when the collector of a transistor (in class A configuration) is biased so that under no-input conditions the output voltage is mid rail. For a 10V supply, the collector would ideally be biased to stand at 5V with no signal present. This allows the largest p-p excursion of a signal before clipping.

It's a little bit more complex than this to achieve optimum q-point results but hopefully this advice should help.

See this http://qooljaq.com/Q-Point.htm for how to set it correctly.

Andy aka
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    There is nothing in the definition of quiescent point that requires the output voltage is mid rail. – Alfred Centauri Mar 19 '13 at 12:35
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    From Wiki also: The quiescent point of operation is typically near the middle of the DC load line. @AlfredCentauri - I'm just trying to give the dude an answer that best suits his question and his knowledge level and sometimes that takes a little reading between the lines. – Andy aka Mar 19 '13 at 12:52
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    @Andy: It might typically be near mid rail, since that allows roughly symmetric swing in both directions from the Q point, but is not part of the definition of the Q point. Not all transistor circuits are meant to produce smooth analog signals, for example. – Olin Lathrop Mar 19 '13 at 13:31
  • For example, if the emitter is at 1V at quiescent and 2V "fully on" the collector can swing from 2V to 10V. The best quiescent point is then 6V at the collector (equidistant between 2 and 10). –  Mar 19 '13 at 14:34
  • I agree but, quoting me above: "It's a little bit more complex than this to achieve optimum q-point results but hopefully this advice should help" – Andy aka Mar 19 '13 at 14:39
  • Also, keep in mind that small signal amplifiers, by definition, operate close to their Q-point and so there might be good reason to choose a Q-point well away from the middle of the large signal operating range. – Alfred Centauri Mar 19 '13 at 19:20
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Although it is certainly true that Q stands for "quiescent," the desired Q-point for a bipolar junction transistor totally depends on what CLASS of amplifier is intended.

Class A amplifiers commonly have the q-point at (or close to) mid-range, because they are intended for the best linear response from a single transistors. But Class A amplifiers are inefficient.

Class B amplifiers typically use two transistors in a push-pull arrangement, and the q-point will be at (or close to) cutoff. Here, the linearity is still fairly good, but the efficiency is much better.

Class C amplifiers will have the q-point well below cutoff. Here, the goal is maximum (or at least very-high) efficiency, and there is intentional clipping of the signal, which is usually RF.

Tom Johnson
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