Transistors
| Transistors | |
|---|---|
| What it does | Uses a small current or voltage to control a larger one, to amplify or switch |
| Invented | 1947, at Bell Telephone Laboratories |
| Main families | Bipolar junction transistors (BJTs) and field-effect transistors (FETs) |
| BJT terminals | Emitter, base and collector; NPN and PNP types |
| More detail | |
| FET terminals | Source, gate and drain; JFET and MOSFET types |
| BJT current gain | Beta (hFE) = collector current / base current |
| Familiar small-signal parts | 2N3904 and 2N2222 (NPN), 2N3906 (PNP), J310 (JFET) |
| RF power devices | Bipolar, MOSFET and LDMOS transistors |
Transistors are semiconductor devices that use a small current or voltage to control a much larger one. That lets a transistor work as an amplifier, making a weak signal stronger, or as a switch, turning a circuit on and off. Invented at Bell Telephone Laboratories in 1947, the transistor has replaced the vacuum tube almost everywhere in radio. A modern transceiver uses individual transistors in its RF stages and final amplifier, and vast numbers more inside its integrated circuits.
Bipolar junction transistors
A bipolar junction transistor (BJT) is a sandwich of three layers of doped silicon, either NPN or PNP. Its terminals are the emitter, base and collector. In an NPN transistor, a small current flowing into the base allows a much larger current to flow from collector to emitter. The base-emitter junction behaves like a diode, so a silicon BJT starts to conduct once its base is about 0.6 to 0.7 volts above its emitter.
The ratio of collector current to base current is the current gain, called beta or hFE. Small-signal transistors typically have a beta of tens to a few hundred: with a beta of 100, a base current of 1 milliamp controls a collector current of about 100 milliamps. Gain falls as frequency rises, and a transistor's transition frequency (fT) is the frequency at which its current gain has dropped to 1, a guide to how high it can usefully amplify. PNP transistors work the same way with all polarities reversed.
Field-effect transistors
A field-effect transistor (FET) controls current with voltage instead of current. Its terminals are the source, gate and drain. A voltage on the gate controls the current flowing through a channel between drain and source, and the gate itself draws almost no current, so a FET has a very high input impedance.
- JFETs (junction FETs) are quiet, simple amplifiers. The J310 is a long-time favorite of builders for VHF preamplifiers and oscillators.
- MOSFETs (metal-oxide-semiconductor FETs) insulate the gate with a thin oxide layer. That gives an even higher input impedance, but the layer is easily punctured by static electricity, so MOSFETs should be handled with a grounded wrist strap. Power MOSFETs are the standard switch in switching power supplies and are widely used as RF amplifiers.
Amplifiers and switches
As an amplifier, a transistor is biased so that it conducts for part or all of each cycle of the signal, giving the familiar amplifier classes:
- Class A conducts for the whole cycle. It is the most linear and the least efficient, used in low-level stages.
- Class AB conducts for more than half the cycle, usually with two transistors in push-pull. It is linear enough for SSB and is the usual choice for transceiver final amplifiers.
- Class B conducts for half of each cycle, with a push-pull pair sharing the work.
- Class C conducts for less than half the cycle. It is efficient but distorts the waveform, so it suits CW and FM but not SSB.
As a switch, a transistor is driven either fully off (cutoff) or fully on (saturation), and it wastes little power in either state. Transistor switches key relays, ground PTT and amplifier keying lines, and drive the transformers in switching power supplies.
RF power transistors
Most 100-watt transceivers run from 13.8 volts and use a push-pull pair of RF power transistors in the final amplifier. Older designs used bipolar transistors; most modern ones use MOSFETs. Inexpensive switching MOSFETs such as the IRF510 are also popular in homebrew HF amplifiers. Because the transistors run hot, they are mounted on large heat sinks, often with fans, and protection circuits reduce power when the SWR or temperature gets too high. Overdriving them or transmitting into a bad mismatch can still destroy them.
LDMOS (laterally diffused MOS) transistors are power MOSFETs built for high power and high gain. Running from supplies of around 50 volts, a single LDMOS device can produce more than a kilowatt at HF, so many solid-state amplifiers reach the US limit of 1,500 watts PEP with one or two of them. Unlike a tube amplifier, a solid-state amplifier needs no warm-up and no tuning, but it must switch in a low-pass filter for each band to remove harmonics, and it depends on protection circuits that a tube amplifier can largely do without.