# How does a transistor amplify voltage ?

#### AlwaysNumber1

Joined Dec 4, 2016
52
Hello everyone !
I am studying transistors right now and have already understood how they are made to amplify current. Could someone explain how does a transistor amplify voltage ?

#### crutschow

Joined Mar 14, 2008
25,989
The amplified current goes through a load resistor placed in the transistor collector or drain node (RL below), and the IR voltage drop across the resistor is the amplified voltage.

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#### AlwaysNumber1

Joined Dec 4, 2016
52
The amplified current goes through a load resistor placed in the transistor collector or drain node (RL below), and the IR voltage drop across the resistor is the amplified voltage.

Yes, I have already seen this circuit many times
Could you please explain how it amplifies in more details ?

#### crutschow

Joined Mar 14, 2008
25,989
It's a simple matter of Ohm's law.
A small AC voltage at the transistor base generates a small base-emitter current which is amplified by the transistor to give a larger AC collector current.
This amplified collector current through the collector resistor creates an AC voltage that is greater than the input voltage, thus the input voltage is amplified.
Don't know how I can explain it any simpler.

#### LesJones

Joined Jan 8, 2017
2,836
Use ohms law and you will see that when the current THROUGH a resistor changes the voltage ACROSS the ends of the resistor changes.

Les

#### AlwaysNumber1

Joined Dec 4, 2016
52
It's a simple matter of Ohm's law.
A small AC voltage at the transistor base generates a small base-emitter current which is amplified by the transistor to give a larger AC collector current.
This amplified collector current through the collector resistor creates an AC voltage that is greater than the input voltage, thus the input voltage is amplified.
Don't know how I can explain it any simpler.
Now I see...
But shouldn't the transistor not conduct at all during the negative cycle ?

#### LesJones

Joined Jan 8, 2017
2,836
In post #02 the capacitor C1 allows the input sine wave to be centered around Vcc x r2/(r1 +r2). so providing the negative peak at that point does not go below the emitter voltage + 0.7 volts the tranisitor will still be conducting.

Les.

#### Doros

Joined Dec 17, 2013
137
What LesJones just described is biasing the transistor, you have to go through biasing in order to amplify signals

#### AlwaysNumber1

Joined Dec 4, 2016
52
In post #02 the capacitor C1 allows the input sine wave to be centered around Vcc x r2/(r1 +r2). so providing the negative peak at that point does not go below the emitter voltage + 0.7 volts the tranisitor will still be conducting.

Les.
Aaaa
So the positive voltage "shifts" the sine wave of the transistor up ?

#### AlwaysNumber1

Joined Dec 4, 2016
52
What LesJones just described is biasing the transistor, you have to go through biasing in order to amplify signals
You mean to select special conditions at a point in the active region of a transistor ?

#### Doros

Joined Dec 17, 2013
137
I do not know if the phrases are correct (english is not my mother language), to raise-put the signal, in the active region of the transistor

#### Sensacell

Joined Jun 19, 2012
2,672
Biasing is about creating the conditions to handle both positive and negative swings of the signal.

For example, an amplifier running from 12 Volts would have a bias point at 6 volts. With no input signal the DC output is 6 Volts.
The output could rise to 12 on positive swings, and as low as zero on negative swings.
In the real world, the output will never be able to go all the way to 12 V or zero V, due to saturation limitations.

The total range the signal can swing is called "headroom" for obvious reasons, you always want to stay within your headroom- otherwise the signal clips!

You always want the transistor to be in the active region to preserve the signal integrity.

#### MrAl

Joined Jun 17, 2014
7,874
Hello everyone !
I am studying transistors right now and have already understood how they are made to amplify current. Could someone explain how does a transistor amplify voltage ?
Hi,

One important point that should be noted is that the word "amplify" suggests that something that already exists is being itself increased directly which is not really the correct view for a transistor amplifier. In other words, when we increase something directly it's like we are adding directly to it, like a bucket of water. With a bucket of water we increase the height of the water by adding more water to the bucket, but with the transistor we do not add to the base current in order to get more current.
What we really do is use the base current to "control' another, separate current, and it seems to 'amplify' because this second current is larger than the base current. We still call it amplification because of the overall operation, but what we are really doing is controlling a larger current with a smaller current, and the two currents are separate.

When we inject current into the base emitter it controls the current from collector to emitter, so we have two separate currents:
1. The base current.
2. The collector current.

Because of the current gain of the transistor, a smaller current can control a larger current. If the base current is 1ma and the gain is 10, then we could see 10ma in the collector. It's that simple really, basically. The more detailed view includes more parameters, but that is a good way to start to think about how the transistor works.

So although we call this amplification, we are really just controlling a larger current with a smaller current by using a transistor.
There are different views on what is really doing the controlling (electrostatic vs charge), but again that is the best way to start.

#### MrSoftware

Joined Oct 29, 2013
1,956
I'm no pro, but I'll take a stab at this. I remember being confused by the same things.

There are (at least?) 2 types of transistors; BJT and FET. The BJT is a current controlled device, the FET is a voltage controlled device. They are both used to control current. Think of them as current valves.

For the BJT, you put a small current into the base to open the "valve" between the collector and emitter. The base current is very small compared to the current that flows through the collector and emitter. The difference is called the gain. The current that you put into the base is added to the current flowing from the emitter. You can increase the current gain by stacking the transistors up in a configuration called a Darlington pair.

The FET is a voltage controlled device. The higher the voltage between gate and source, the more current can flow between the drain and source. So to open the "valve" between the drain and source, you supply more voltage to the gate. The gate is basically a capacitor; once it reaches a steady voltage, essentially no current will flow into it. And to turn the FET off, you have to discharge the gate.

FETs are available with logic-level gates. In other words, you can have say 60v between source and drain, and have the FET fully turned on by only supplying a few volts to the gate. In this case you're not really amplifying a voltage, but you are controlling a 60v supply with only a few volts to the gate.

In both cases, if the transistor is connected to a load then ohms law will give you the voltage across the load.

Edit: For the FET, there are N-channel and P-channel, and for the BJT there are NPN and PNP. Basically they work opposite (compliment) each other. I can't do justice to the explanation, but Google the terms and you'll get tons of tutorials.

References: https://en.wikipedia.org/wiki/Transistor#Types

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#### spinnaker

Joined Oct 29, 2009
7,835
I'm no pro, but I'll take a stab at this. I remember being confused by the same things.

There are (at least?) 2 types of transistors; BJT and FET. The BJT is a current controlled device, the FET is a voltage controlled device. They are both used to control current. Think of them as current valves.
Vacuum tubes, the predecessor or transistors where often referred to as "valves" .

#### MrAl

Joined Jun 17, 2014
7,874
Vacuum tubes, the predecessor or transistors where often referred to as "valves" .
Hi,

Yeah i think that might be a little country specific? I have to laugh a little whenever i hear that but then again it makes a lot of sense because it does act like a water faucet that is a valve that controls the flow of water only the tube controls the flow of current.

#### Papabravo

Joined Feb 24, 2006
14,833
Rather than thinking about transistors "amplifying" voltage or current, it might be helpful to think about them as "copying" something. For example:

The transistor takes a small base current and makes a larger copy of the base current in the collector circuit.

#### BR-549

Joined Sep 22, 2013
4,938
We don't amplify current. Or voltage. We amplify the signal. The current or the voltage is just the carrier of the signal.

With a BJT, the input signal is a little bit of current with a wiggle on it. The wiggle is the signal....the little current carries it into the transistor.

The electrical structure of the transistor ....transfers the little wiggle....onto a larger output current carrier.

A FET uses voltage for the wiggle carrier.