Transistor Amp Amplification: DC vs AC | Frequency Impact

[kjell]

Homework Statement

I'm interested in the difference between the way a transistor amplifier amplifies with DC and with AC. I know that a transistor amplifier connected to a DC voltage source will have a larger gain with a bypass capacitor than without. Is this also the case when connected to AC? Is the gain the same on DC as it is on AC? How does frequency figure into things?

 

[vk6kro]

Without the emitter bypass capacitor, and without the series coupling capacitors, the gain for AC and DC is close to Rc / Re.


However, for AC, the effect of the emitter resistor is much reduced with the bypass capacitor and the gain is much higher.

Also, of course, AC can pass through the coupling capacitors while DC can't.

 

[waht]

The transistor also has an internal emitter resistance re, which is usually in 10 ohms range.

if the bypass emitter cap shorts the AC, the only remaining resistance becomes re

and the max possible gain comes out to -Rc/re

 

[kjell]

Thanks for the informative comments.

I have one other related question. I know that the gain is higher with a decoupling capacitor but is there a difference in the frequency response between a setup with a decoupling capacitor and a setup without a decoupling capacitor?

Is it reasonable to say that the transistor amplifier has non-linear frequency dependence?

 

[kjell]

Without the emitter bypass capacitor, and without the series coupling capacitors, the gain for AC and DC is close to Rc / Re.

I did some checking and it looks like the gain is Rc / re WITH the bypass capacitor and
Rc / (re + Re) WITHOUT the bypass capacitor.

 

[vk6kro]

I did some checking and it looks like the gain is Rc / re WITH the bypass capacitor and
Rc / (re + Re) WITHOUT the bypass capacitor.

Yes, that is right.

Without the bypass capacitor, the gain depends mainly on the ratio of RC/RE if rb is small enough to be negligible.
eg if RC= 50 K and RE= 10 K and rb =150 ohms, gain = 50 / 10.15 = 4.926
Without considering rb, gain = 50 / 10 = 5... very close to the same gain and it doesn't depend on the transistor gain provided the gain is 50 or more.

Incidentally the bypass capacitor should have a low enough reactance so that it is negligible compared to the emitter resistor's resistance at the lowest frequency to be amplified. Otherwise, the gain will drop back to the above situation.

Is it reasonable to say that the transistor amplifier has non-linear frequency dependence?
Yes, it is reasonable. All amplifiers have limited bandwidth, meaning their gain cannot be equal at all frequencies.