Understanding Transistor Voltage Amplification

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In summary, the transistor amplifies the voltage at the collector when the collector current increases.
  • #1
Sigurdsson
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Homework Statement


I've been reading about transistors now for a while and there is something that doesn't add up. It's about how the collector in a transistor amplifies the output voltage.

williamson-labs.com said:
As the voltage at the generator goes more positive; the base current increases; the collector current increases; the voltage drop across the collector resistor increases; and the voltage at the collector goes less positive or lower.

And then they say...

[QUOTE="williamson-labs.com]When operating with a collector resistor (RL): the output voltage from the collector is an amplified voltage.[/QUOTE]


What I was thinking is that when the collector current increases (amplification is usually around 100) the voltage is amplified, hence a high output at the collector. But high current at the collector would mean a big voltage drop in it's resistance and hence low output voltage in the collector.

So my question is basicly...is it low current or high current in the collector that amplifies the voltage there?
 
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  • #2
It doesn't make sense to say there is amplification "in the collector". The whole transistor is involved.
The transistor is basically a device that amplifies current. The collector current is many times the base current. The input signal varies the base current within some reasonable range, and the collector current varies a much larger amount in response. When the collector current passes through a resistor, you get a varying voltage across it. Input and output resistors can be chosen so the variation in the voltage across the output resistor is much larger than the variation in the input voltage.
 
  • #3
Sigurdsson said:
What I was thinking is that when the collector current increases (amplification is usually around 100) the voltage is amplified, hence a high output at the collector. But high current at the collector would mean a big voltage drop in it's resistance and hence low output voltage in the collector.

So my question is basicly...is it low current or high current in the collector that amplifies the voltage there?

Both of your quotations are correct. There are two things to keep in mind that might help you make sense of all of this:

(a) The output waveform (voltage as measured at the collector) is inverted, as compared to the voltage measured at the generator. So if the generator contains a sine wave component, you will see a sine wave at the collector too, except it will be 180o out of phase.

(b) There is still amplification. A small "wiggle" in voltage at the generator produces a comparatively large wiggle in the voltage at the collector. Granted, it is in the opposite direction, but it is much bigger wiggle.

Essentially, the amplification they are talking about is AC amplification. The DC voltage of the collector doesn't matter in AC amplification because you can always DC bias it accordingly (you need to do this anyway to minimize saturation).

In short, the AC gain (aka amplification) of BJT transistor in a common-emitter configuration is typically negative with magnitude greater than 1.
 
  • #4
I think I got it...

http://www.play-hookey.com/digital/electronics/images/invert.gif [Broken]

I was thinking with the output above the resistor in this circuit. If we have, let's say 5 volts, at v+ and there is a massive voltage drop at the RL resistor. Then there has to be large voltage at the other end.

[QUOTE="williams-labs.com]...and the voltage at the collector goes less positive or lower.[/QUOTE]

Like collinsmark pointed out the signal is inverted. So when they are saying in the above quote that the voltage at the collecter goes less positive, it's amplitude is increasing...while I thought decreasing.

*facepalm*


Correct me if I'm wrong, otherwise cheers guys!
 
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1. What is transistor voltage amplification?

Transistor voltage amplification is the process of increasing the voltage of an electrical signal using a transistor. This is achieved by applying a small input voltage to the base of the transistor, which then allows a larger voltage to flow through the collector and emitter. Essentially, the transistor acts as a switch or a valve for the flow of electrical current, allowing for amplification of the signal.

2. How does a transistor amplify voltage?

A transistor amplifies voltage through a process known as the "transistor action." This involves the movement of electrons from the emitter to the collector, controlled by the voltage applied to the base. When a small voltage is applied to the base, it allows a larger amount of current to flow from the emitter to the collector, resulting in an amplified voltage.

3. What factors affect transistor voltage amplification?

There are several factors that can affect the voltage amplification of a transistor, including the type and properties of the transistor, the voltage and current levels, and the circuit design. Additionally, temperature, noise, and other environmental factors can also impact the performance of a transistor and its ability to amplify voltage.

4. How is the voltage gain of a transistor calculated?

The voltage gain of a transistor is calculated by dividing the output voltage by the input voltage. This is typically expressed in decibels (dB) and can be calculated using the formula: Voltage gain (dB) = 20 log (Vout/Vin), where Vout is the output voltage and Vin is the input voltage. The higher the voltage gain, the more amplification the transistor is providing.

5. What are some common applications of transistor voltage amplification?

Transistor voltage amplification is used in a wide range of electronic devices and systems, including audio amplifiers, radio transmitters, and computers. It is also commonly used in power supplies, sensors, and control circuits. In general, any electronic system that requires amplification of a voltage signal may make use of transistor voltage amplification.

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