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In a transistor radio, what gets amplified? Is it voltage? Is it possible for a transistor to amplify voltage? I looked at the transistor Wikipedia web page, but it didn't say anything about whether a transistor amplified voltage.

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  • \$\begingroup\$ Where else did you look. There are thousands of books and probably tens or hundreds of thousand on-line resources on basic transistor amplifier theory. \$\endgroup\$
    – Transistor
    Commented Sep 23, 2019 at 20:11
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    \$\begingroup\$ Transistor (BJT) is amplifying current if biased appropriately. But since current and voltage are tightly related, a voltage amplifier can be built around it. \$\endgroup\$
    – Eugene Sh.
    Commented Sep 23, 2019 at 20:11
  • \$\begingroup\$ A radio antenna might provide picowatts \$ (10^{-12} \$ W) of RF power to the radio's front end. The audio that emerges to the speaker might be tens of milliwatts (0.01 W). Power amplification is what really matters. \$\endgroup\$
    – glen_geek
    Commented Sep 23, 2019 at 20:21
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    \$\begingroup\$ If the wikipedia article doesn't state that voltage amplifiers exist, that sounds like an article sorely in need of expanding. \$\endgroup\$
    – Hearth
    Commented Sep 23, 2019 at 20:25
  • \$\begingroup\$ @Hearth Adding voltage amplifiers to that article would make the article wrong, not expand it. Ideal voltage amplifiers don't exist, nor can they. All amplifiers are some kind of transconductance amplifier. Amplifiers with voltage gain (a 'voltage amplifier') is still amplifying current as well, because they're transconductance amplifiers. A voltage amplifier would have infinite input impedance, since it would require no current (it only amplifies voltage!). In the real world, every amplifier requires input bias current, and is thus not a voltage amplifier. \$\endgroup\$
    – metacollin
    Commented Sep 23, 2019 at 22:47

3 Answers 3

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A transistor can amplify current, or voltage, or both, depending on how it is configured in the circuit. A bipolar transistor configured as 'Common Collector' amplifies current, but not voltage. When configured as 'Common Base' it amplifies voltage, but not current. In 'Common Emitter' mode it can amplify both.

Ultimately the goal is to amplify the power of the signal, so most transistor amplifiers use 'Common Emitter' mode because power = voltage x current so amplifying both can provide greater amplification. However sometimes one of the other modes is more appropriate.

Common Base mode is sometimes used at high rf frequencies due to its wider bandwidth and greater stability. Here is an example of the 'front end' of an FM tuner, showing a junction FET in 'Common Gate' mode and an NPN bipolar transistor in 'Common Base' mode.

enter image description here

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In a transistor radio, amplitude of the received power, or put another way, a fixed ratio of voltage and current is what is amplified. Using an AM/FM radio as an example, if the radio's receiver has its resonant frequency changed (tuned) to match that of a broadcasting station that is within range, the antenna and receiver will resonate with the broadcast frequency, allowing a minute amount of signal power (typically measured in dBm, or dB with 0 dB equalling 1 milliwatt) to be received by the radio.

This will be very tiny, on the order of 10s of picowatts for something like a strong FM radio signal, or in more extreme cases, like GPS receivers, this can be as feint as hundreds of attowatts, (picowatt = 1 million attowatts).

Broadcast FM radio stations with a ~30 mile range often transmit at 80dBm, or 100kW. But far-field electromagnetic radiation falls off with distance squared, as it spreads out over what is effectively a surface. If you imagine a sphere centered on the broadcast antenna, and the distance the radius, then the surface area of the sphere increases with the square of the radius. (Note: the actual radiation pattern is usually not sphere shaped, but the point is, it spreads out fast with distance).

So even with 100kW of electromagnetic radiation ejecting out of this broadcast antenna like an omnidirectional riot hose, by the time it is picked up by your radio many miles away, it has spread out to almost nothing. GPS is much much further, and it spreads out that much more.

This received electromagnetic radiation causes electrons in the antenna to wiggle back and forth slightly, causing a small AC voltage in the antenna. The wiggling is also AC current, but what matters is received power. Power is voltage • current (more complex for AC but instantaneously, power is still voltage • current). Depending on the impedance of the antenna and receiver, this will result in more or less voltage vs current, with higher impedance resulting in higher induced voltage but less current.

But there is little point in making any distinction here, which is why I say that the RF receiver amplifies the amplitude of the power. It takes the tiny wiggles picked up by the antenna and makes them bigger wiggles. Typically, the wiggles need to be large enough (in amplitude) to drive the demodulator, which will output the decoded audio signal to an audio amplifier, which is a power (as in watts) amplifier that drives a loudspeaker or perhaps merely headphones.

The RF amplifier couples to an (ideally) matched input or output impedance, meaning whatever the impedance of the antenna/bandpass receiver circuit will be matched to the impedance of the demodulator, and so the transistors in the RF amplifier are amplifying a fixed ratio of voltage and current. You can characterize them as amplifying voltage, and you can characterize them as amplifying current, both are just that: a characterization, a convenient but incomplete point of view. Fundamentally, the transistors in an RF amplifier are amplifying power. To increase current through a given impedance, you must increase voltage.

On a low level, bipolar junction transistors are transconductance amplifiers. This means they take input voltage and output a larger current. If the gain is 10, 100mV would produce 1A out. However, because for BJTs the voltage needed to achieve this gain is very low, about 26mV at room temperature, and the base has a exponential voltage/current relationship like a diode such that enough voltage to turn the transistor on will always result in some measure of base current as well, it is convenient to approximate their functionality as current amplifiers. They take a small base current and make it a larger collector current. But in reality, a small voltage results base current which makes the path from collector to emitter more conductive. You can bias a BJT so it behaves like a voltage amplifier, current amplifier, power amplifier, and just about anything in between.

MOSFETs are also transconductance amplifiers. So are vacuum tubes. True voltage amplifiers do not exist, nor do current amplifiers. All amplifiers amplify both, the rest is merely a matter of how you chose to look at the circuit.

In RF amplifiers, the transistors are usually biased to match impedance, meaning their gain is to power, and the voltage and current is amplified in a fixed ratio to realize that.

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Lets look at a circuit

schematic

simulate this circuit – Schematic created using CircuitLab

What is the gain? ignoring any external load (on Vout) and the effect of 91Kohm in parallel with 2.5K, the gain is Rc * transconductance

Gain = 2.5K * 0.0385 = 96X

Gain is also 2.5K / 26 ohms, which I know a bipolar has for 1/transconductance at exactly 1mA. At 0.1mA, its 260 ohms. At 10uA, its 2,600 ohms.

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