Bias voltage of transistor

Thread Starter

Electronic_Maniac

Joined Oct 26, 2017
252
I would like to understand the purpose of biasing the transistor and why It has to be halfway between Vcc and ground?
And which terminal of the transistor (Base, collector or emitter) should be between halfway between Vcc and ground while biasing.
An analogy would help a lot. Please
 

MrChips

Joined Oct 2, 2009
20,231
I would like to understand the purpose of biasing the transistor and why It has to be halfway between Vcc and ground?
And which terminal of the transistor (Base, collector or emitter) should be between halfway between Vcc and ground while biasing.
An analogy would help a lot. Please
You have taken this out of the context of LVDS.
Specify your particular context.

For a linear Class-A amplifier biasing the output node of the amplifier to (Vcc - GND)/2 gives you maximum dynamic range, assuming rail-to-rail linearity.
 

Thread Starter

Electronic_Maniac

Joined Oct 26, 2017
252
You have taken this out of the context of LVDS.
Specify your particular context.

For a linear Class-A amplifier biasing the output node of the amplifier to (Vcc - GND)/2 gives you maximum dynamic range, assuming rail-to-rail linearity.
This is not related to LVDS i think. Could you explain what you mean by maximum dynamic range when biased at Vcc/2?
 

MisterBill2

Joined Jan 23, 2018
5,194
I would like to understand the purpose of biasing the transistor and why It has to be halfway between Vcc and ground?
And which terminal of the transistor (Base, collector or emitter) should be between halfway between Vcc and ground while biasing.
An analogy would help a lot. Please
For starters, biasing so that the base is held half way between saturation and cutoff is used in class "A" amplifiers. Most transistor circuits are not class "A" amplifiers, and so they are not biased there. For a switching circuit the transistor is either biased below starting to conduct, or else biased at a level of conduction adequate for the application.
So the question needs a lot more details as to what the transistor is wanted to be doing. Otherwise the best answers given will all be guesses, wasting everybody's time.
 

Audioguru again

Joined Oct 21, 2019
886
I simulated a transistor circuit. It has an emitter resistor R5 to reduce the bias change caused by transistors that have a different vbe. The peak voltage across R5 is about 1V so the maximum output swing will be 9V - 1V=8V. Therefore the base voltage is biased so that the collector voltage idles at about 5V so the output can swing 4V up to 9V and down 4V to 1V.
With high gain a transistor produces a very distorted signal.
With some negative feedback the distortion is very low as is shown in this simulation:
 

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MisterBill2

Joined Jan 23, 2018
5,194
The negative feedback is provided by removing the capacitor across the emitter resistor. Note that the gain is greatly reduced along with the distortion. That is the compromise .
 
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