Hi MrAl,
I think it would be easier for you sir to explain if you know how I understand the circuit so you could point out what am I not understanding well.
1. The voltage across the capacitor, C1, is the same as the input voltage.
2. The voltage across the resistor, R, is the total input voltage minus the zener diode voltage rating.
3. The voltage that can be measured at V+ is the zener voltage minus the base emitter voltage of the transistor.
Questions:
1. Isn't it that the input voltage here is also the power supply voltage?
2.
How can we see a voltage gain if we'd end up with the full power supply voltage? If the input voltage is the same with that of the output voltage, then I am thinking sir that there is no gain.
meowth08
Hi again,
The voltage gain YOU are talking about is the ratio of the output voltage to the power supply voltage Vo/Vs..
The voltage gain that I am talking about is the ratio of the output voltage to the zener voltage Vo/Vz.
The voltage gain from the standpoint of the transistor alone is based on the voltage gain looking at it as an amplifier. The common collector configuration has a gain of slightly less than 1, we'll call it 1. If you have 10v power supply and 5v output, that's a gain (according to your intended view) of only 1/2, yet the gain of a voltage follower circuit has a gain of 1 (common collector). So you see when we talk about the voltage gain of the transistor we talk about the input of the transistor, not the input power supply. The input power supply is not the input it is just the power supply. It could be 10v, 15v, 20v, 100v, etc., but we'd still see a voltage gain of 1 because the zener is (close to) 5v and the output is 5v.
The input power supply is not the input, it's just the power supply, just like any other transistor amplifier, and in the voltage regulator the theory of operation is such that the transistor acts as an amplifier. In common collector config, it operates as a current amplifier so there is current gain usually greater than 1, but the voltage gain is 1 or a little less.
In common emitter config the voltage gain is usually more than 1 and so is the current gain, so we have both voltage and current gain. In the common collector config we just have current gain and no voltage gain (we all a voltage gain of 1 "no" voltage gain).
Another example where we see the property of a voltage gain of 1 is with an op amp voltage follower. We use a very small input current and get a larger output current but the output voltage is the same as the input voltage so we only see a voltage gain of 1. The power supply just supplies the power to the circuit that's it.
In a voltage regulator we usually talk about other properties like 'regulation' with input line change, and 'regulation' with output load change, like that. But for the transistor itself in the circuit, we often talk about the gain of the transistor itself as being either a voltage gain or a current gain or both. It's a property of the transistor itself and it's configuration and doesnt matter what the input supply voltage is.
Sometimes we talk about a regulator circuit as having a certain "input voltage", where we mean that we use a 12v input supply and even what the minimum "input" voltage should be (referring to the input power supply voltage) but that's not the same as when we view the transistor itself and the transistor configuration. So when we talk about the voltage 'gain', we are not talking about the input power supply voltage to output voltage we're talking about how the transistor itself is behaving.
So when we talk about the whole power supply we say the 'input' voltage is the power supply input, but then we dont talk about any gain. When we talk about the transistor itself and the way it is used, we talk about the gain of the transistor itself and not the whole power regulator.
So when i say the gain is 1, it means the transistor gain is 1, not the whole circuit gain.