On July 26, 2015 at 20:51, amirm said...
Ali, there are some mistakes in your explanation. The voltage drop across a bipolar transistor in an amplifier is Vce, not Vbe. There are two junctions there and therefore the drop is 1.2 volts, not 0.6 and certainly not some small leakage voltage.
There are three voltage drops in a transistor: Vbe, Vce, Vcb, which are you referring to? The only one that you can actually get off the top of your head is Vbe because for forward active mode, or in other words amplification mode, it HAS to be ~0.7V. The voltage drop across Vce has to be GREATER THAN Vcesat which is ~0.3V but that isn't a set number. Depending on how much current your drawing and the circuit itself that value will be different. If you take a look at any document explaining the operating regions of a BJT you'll see that there are 4 modes and that only forward active can be used for amplification. The two things I mentioned above are required for the BJT to be in that mode.
|FETs are used in small minority of power amplifier designs. Vast majority use |bipolar transistors.
This is true, and I mentioned that when I said FET's drawback is in power, but FET's are common for input stages. In fact, they're almost better in every case except for power and that is usually only one stage in an amplifier. Even the simplest op amps have 4-5 stages of amplification.
The impedance as seen by the load is also in series as Ernie explained. This is shown in the equiv. circuit of any amplifier:
As you see, Ro (output impedance) is in series with the load.
Also, sorry, I should've been more clear. Yes you can model it that way at DC. However, at AC, all DC sources are shorted, thus putting load resistances in parallel with the output resistance. Please look at page two of the document below. Input and output impedances as well as gain are all small signal values which are all given at AC, not DC.
[Link: whites.sdsmt.edu]
