Hi,

This might come across as naive but I'm really confused about something with transistors and I would like some insight. I've been reading books and sites online and my questions don't seem to get answered. I know that one of the uses for a transistor is for current gain. What does this mean exactly in relation to the voltage source when the transistor is in saturation? Does this mean that if my voltage source can only source something like 20 milliamps by itself, can I connect a transistor and then be able to source a higher current like 50 mA? In other words, can I saturate a transistor then be able to connect a load that has a higher current than the voltage source can allow? Is that what is meant by current gain?

Thanks

 

No. The current gain refers to the ratio of collector current relative to the base current.

 

When a BJT transistor turns on all the way it is in saturation. A loose rule of thumb is it takes 1/10 of the current through the BE to saturate the CE. At saturation you can usually treat a transistor as a closed switch, very digital. So 1ma through the BE will allow up to 10ma through the CE. Saturation is when the CE is less than 0.2V, it can be much less.

When in the analog region the old formula Ic=β*Ib works pretty well.

 

You must know that the term "gain" shouldn`t be misunderstood.
For example: The function of a simple transistor gain stage can be described as a converter that converts dc power (from the supply) into ac power.
And "gain" is "only" defined as the ratio of ac ouput voltage to ac input voltage.
(and the "source" of the amplified ac voltage is the dc supply).

 

The gain refers to the input.

Voltage source is not an input.

The current at the Base of BJT transistor is the input. So lets say the current at the Base is 1 mA. The voltage source is capable of providing 20 mA. Gain is calculated by dividing output by input. Gain=20 mA/1 mA=20. So for this specific example the maximum gain is 20. The maximum gain scenario is that you are putting in 1 mA, you are getting out 20 mA. You just gained 19 mA. Where does that extra current come from? From voltage source.

 

Thanks for the answers everyone, shteii, your answer really makes everything make sense to me.

Another question: What determines the maximum output current of a voltage source?

 

The maximum output current is determined by the total series resistance in the circuit, including both internal and external resistances, using Ohm's Law, I = V/R.

For example, if the open loop voltage source is 10V and the total series resistance is 5Ω, then the maximum current is

I = V/R = 10V/5Ω = 2A.

Edit: I should add of course before another member jumps on me that this only applies in the case when R is constant. In a non-linear system, R is not constant and therefore the result cannot be determined by simply applying Ohm's Law.

 

And, in an actual circuit like a common emitter amplifier, you select a transistor with a gain higher than you need at the required voltages/currents. The resistors in the circuit actually determine the gain.
This is because transistor gain is not one exact value for any given device or for a batch of the same device, but is specified in a range depending on voltage and current.

 

.... So lets say the current at the Base is 1 mA. The voltage source is capable of providing 20 mA. Gain is calculated by dividing output by input. Gain=20 mA/1 mA=20. So for this specific example the maximum gain is 20.

I am afraid, the above sentences could create some misunderstandings. Therefore, I like to add:
The current gain is determined by the transistor unit only.
It is a transistor parameter.
And the collector current is set by the applied base-emittervoltage only (as long as the transistor is operated in its quasi-linear range)

 

And, in an actual circuit like a common emitter amplifier, you select a transistor with a gain higher than you need at the required voltages/currents. The resistors in the circuit actually determine the gain.
This is because transistor gain is not one exact value for any given device or for a batch of the same device, but is specified in a range depending on voltage and current.

"
....depending on voltage and current..."

I think, it is helpful to be somewhat more specific.
In a transistor amplifier we have to distinguish between
*Base-emitter voltage Vbe, collector-emitter voltage Vce, DC supply Vcc.
*Base current Ib, collector current Ic
* current gain of the BJT, current gain of the whole circuit (stage) and voltage gain of the stage.

For my opinion, any explanation should use the relevant names - otherwise, misunderstandings cannot be avoided.

 

Everyone, you've all been so helpful! Thank you for helping a beginner out.