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Transistor As Amplifier / Switch

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crutschow

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To work as an amplifier or switch, a device requires power gain. Bipolar transistors amplify current, i.e. a larger collector current is controlled by a smaller base current. A field-effect transistor controls the drain current by a voltage change on the gate.

In either case the power you can control at the collector/drain is larger than the base/gate power at the input, thus giving power amplification.

If the transistor is biased properly, and you vary the base/drain from an AC current/voltage, then then collector/drain current will vary in proportion and you have an AC amplifier.

If you change the base/drain current/voltage a large enough value you will cause the collector/drain current to go from full on to full off, creating a switch.

The maximum gain you can obtain from one stage is determined by the gain of the transistor (β for a bipolar transistor, and transconductance for the FET). These vary signicantly from unit to unit so, to negative feedback is often used in amplifier circuits to reduce gain variation.

Don't understand your question about "Variation in Q-Point due to β in FB".
 

EN0

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Here is a audio amplifier circuit I built, and using a ceramic earphone at the output and a microphone at the input, you can hear your voice. Note that there are different configurations for transistors, Common-Emitter, Common-Collector (The configuration I used), and Common-Base. Common-Emitter is probably the best configuration for Class-A audio amps, but common-collector works pretty well. The two resistors at the base form a voltage divider and according to the equation with these resistors: R3/R2+R3 x Vin = Vout and so we get 4.5V at the base. Now, because of the Voltage base-emitter drop (Vbe), we subtract 0.7V (usually for a silicon transistor) to find the voltage at the emitter. This enables the transistor to "turn on" and the biasing resistors help with this. β is the gain of the transistor and that can be calculated as follows: Ic / Ie.
 

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audioguru

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ElectroNerdy's common-collector circuit is not an amplifier because it has no voltage gain. It is an impedance converter with a fairly high input impedance and a fairly low output impedance.
A common-emitter circuit is an amplifier with plenty of voltage gain, a medium input impedance and a medium output impedance.

The beta of a transistor is its current gain, not its voltage gain. It is not Ic/Ie but instead is Ic/Ib.
 

EN0

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ElectroNerdy's common-collector circuit is not an amplifier because it has no voltage gain. It is an impedance converter with a fairly high input impedance and a fairly low output impedance.
A common-emitter circuit is an amplifier with plenty of voltage gain, a medium input impedance and a medium output impedance.

The beta of a transistor is its current gain, not its voltage gain. It is not Ic/Ie but instead is Ic/Ib.
Man, I made some big mistakes. Thanks for correcting me Audioguru!
 

crutschow

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ElectroNerdy's common-collector circuit is not an amplifier because it has no voltage gain.
I know the common definition of an amplifier is a voltage amplifier but, in the general sense of the term, you can also have a power amplifier (for example RF amps are often rated for power gain, not voltage gain). And the common-collector (emitter-follower) circuit qualifies as a power amplifier.
 
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audioguru

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An emitter-follower output is in every audio amplifier but they all use more transistors to provide voltage gain.
This is an audio amplifier without any voltage gain, not an RF amplifier.
 
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crutschow

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An emitter-follower output is in every audio amplifier but they all use more transistors to provide voltage gain.
This is an audio amplifier without any voltage gain, not an RF amplifier.
Of course they do and of course it's not. But that wasn't my point.
 

EN0

Member
That's funny Audioguru, I wonder why they named it "Common-Collector Amplifier"? You're saying it should be something like "Common-Collector Impedance Buffer."
 

Roff

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That's funny Audioguru, I wonder why they named it "Common-Collector Amplifier"? You're saying it should be something like "Common-Collector Impedance Buffer."
It's a current amplifier. Put in a little signal current, get a lot of signal current out.
 
If the transistor is biased properly, and you vary the base from an AC current/voltage, then then collector current will vary in proportion and you have an AC amplifier.

If you change the base current a large enough value you will cause the collector current to go from full on to full off, creating a switch.
How does varied collector current cause amplification?
Where exactly should i input the Signal for an BJT Amplifier / n where do i get output?

may i have a diagrammatic representation of BJT as a switch?
 
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