Here is a simple explanation of how amplification is achieved. I will use the vacuum triode as an example, but the same theory applies basically to the diode, other tube types (pentode, tetrode, etc.), and to the transistor.
The main difference with the transistor, especially as it is employed in computer chips is as a switch. It is either switched full on or full off, giving a high and a low (1 or 0), whereas in amplifier circuits, it is continually biased along the linear trans-conductive part of the conduction curve so that equal bias changes on the input results in an equal conduction level on the output.
The basic concept of an amplifier then is to effect a LARGE change/swing in output for a small change/swing in input. The basic vacuum tube amplifier:
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You have an input side where a small, line-level voltage signal is fed in, which controls
a much larger output voltage and current on the output side.
This is accomplished with three (tri) basic parts:
- The cathode which is usually doped with some material which when heated, emits electrons, so is negatively charged (often at ground potential).
- The anode side (often called B+) which holds a highly positive voltage (may be 300-400 VDC above the cathode).
- And the grid, which carries a small negative charge, connected to the input signal (decoupled from DC from the earlier input stage with a decoupling capacitor).
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In the heated cathode design, the cathode is heated up to emit its electrons. It is this heating up process which is why tubes must "warm up" before operating.
As the cathode heats and emits electrons, they form a "space charge" or cloud of electrons, which are the "fuel" of the circuit, held between the very negative cathode and the slightly less negative but still negative grid. Since like charges repel, the space charge is held here and all electrons are repelled from flowing to the anode. The volume is "down" and the amplifier emits no sound.
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Then when the input AC signal is applied to the control grid, the bias of the grid is biased more or less negative, increasing and decreasing the repulsive force of the grid. When the grid signal is biased less negatively enough, eventually, its repulsive force begins to be overcome by the strong attraction of the highly positive anode and electrons escape the space charge, flow past the grid and make it to the anode.
In this way, small variations in the low level input signal close to the cathode effect large swings in electron flow through to the anode (sometimes called The Plate). These cause large voltage swings dropped across the output resistor and current flows back to ground, thus, a tiny input signal in millivolts is /amplified/ to create an analogous but much larger output signal.
A diode is the same except it just has the two poles and the bias determines whether it is closed (shorted) to allow conduction or biased open to block all conduction.
Same with a transistor. The principle is pretty much the same except a semiconductor junction replaces the grid and much lower voltages are used. This is the basic principle behind all amplification.
