Basic transistor amplifier question ?

[crazyjohn]

Are transistor amplifier configurations mainly used in electronics for amplifying an ac signal so that it changes phase or amplitude on the output?

Also are transistor amplifiers used much for dc only in active region mode (not being used as a typical switch) ?

 

[ark5230]

Transistors are used both for AC and DC signal amplification, change of phase amplitude etc are considerations for design based on the requirement.

 

[chuckey]

Most requirements are for, say a hi fi amplifier, where you want to amplify 50mV from the microphone or pickup to a high power level say 50W. So this is an amplitude requirement. I have never heard of a requirement for an amplifier that does not change the signal's amplitude but only its phase.
I would say no, because DC is just that. A small DC voltage from a probe may need to be amplified so it can be measured using a meter. But if the DC does not change then there is no point in measuring it. There are a lot of amplifiers that are used where the DC stability is of great importance, so in the above example if the DC level of the output of the amplifier drifted with temperature, reading the meter, one would be unsure if the change of reading was due to the signal changing or was because the amplifier was drifting.
Frank

 

[crazyjohn]

What sort of applications would a dc only transistor amplifier be used ?


Whats the advantage of using a opamp to amplify an ac microphone signal vs a transistor amplifier that does the same thing?

 

[chuckey]

Thermocouples with a direct temperature reading on a meter.
An opamp has a much higher gain then a single transistor, so a 1$ opamp can replace 4 transistors + 10 resistors +6 capacitors + all their wiring and possibility of going wrong. Its a no-brainer, use a ready made amplifier in a can!.
Frank

 

[crazyjohn]

Couple of questions:

Are most transistor amplifier circuits dc coupled with ac in and out resulting in amplification on the ac signal on the output ?

Would you say that dc only transistor configurations mainly are used to operate as a switch and not in the linear active region ?

 

[chuckey]

"Are most", can't say. Low level discrete component amplifiers are normally AC coupled between stages aa are high power RF amplifiers. Most Audio high power amplifiers are DC coupled with AC coupling on the input and output.
NO, DC coupled transistor amplifiers are used for DC amplification or high power amplifiers which are extremely linear.
All logic integrated circuits operate outside their linear region's, in the sense that an input "1" causes the output to saturate and so as to get close to an output "1" or "0" as possible. There are logic elements that have "Schmidt" inputs which are switches in the sense that there is no input voltage that does not represent a "1" or a "0" i.e. no linear region.
Frank