LearnTronics

History

The Triode

A small electrical signal learns to control a much larger current

The vacuum-tube diode could rectify and detect a signal, but it could not amplify one. To make electronic communication truly powerful, engineers needed a device in which a weak electrical signal could control energy supplied from a separate power source.

This is one of the central ideas of electronics:

a small signal does not have to provide all of the output power — it can control a larger flow of energy.

Lee de Forest adds a grid — 1906

In 1906, American inventor Lee de Forest added a third electrode to the vacuum-tube diode. He placed a wire grid between the electron-emitting filament and the plate. His three-electrode tube became known as the Audion.

A three-electrode tube is a triode: cathode, control grid, and plate. The crucial discovery was that the voltage on the grid could strongly influence how many electrons reached the plate.

The triode's three active electrodes:

Cathode — heated so it emits electrons.
Control grid — placed in the electron stream; a small voltage here controls the flow.
Plate/anode — normally held positive so it attracts electrons and carries the larger plate current.

How the grid controls current

The grid sits very close to the cathode. If the grid becomes more negative relative to the cathode, it repels electrons and reduces plate current. If it becomes less negative, more electrons can pass through the openings in the grid and reach the plate.

more-negative grid → fewer electrons reach plate → less plate current
less-negative grid → more electrons reach plate → more plate current

Because the grid can control a comparatively large plate current while drawing little current itself under normal operating conditions, a weak input voltage can control a much larger amount of power coming from the tube's plate supply.

That is amplification

Put a changing signal on the grid and the plate current changes with it. Pass that changing plate current through a load resistor, transformer, headphones, loudspeaker transformer, or another circuit, and a larger version of the signal can be produced at the output.

input signal at grid → controls plate current → plate supply provides the energy → larger output signal

The energy in the larger output does not come from nowhere. It comes from the tube's power supply. The small grid signal merely controls how that stored or generated energy is delivered to the load.

The beginning of active electronics

A resistor can limit current. A capacitor can store electric field energy. An inductor can store magnetic field energy. A diode can steer current in one direction. But the triode can use one signal to control another circuit and produce gain. That makes it an active device.

This changed what electrical systems could do. Weak radio signals could be amplified. Audio signals could drive headphones and loudspeakers. Signals could be passed from one stage to another, becoming stronger each time. Long-distance communication became much more practical.

The earliest Audion tubes and the later high-vacuum triodes were not identical in behavior. Understanding of the device, vacuum technique, electrode construction, and reliable amplification all improved rapidly during the following years. The important conceptual step was the same: the grid gave an electrical signal control over plate current.

Gain — a useful new idea

Measurement trail 23 — voltage gain

One simple way to describe an amplifier is to compare the size of its output signal with its input signal.

voltage gain = output signal voltage / input signal voltage

A simple amplifier example

If a 0.25 V changing signal at the input produces a 5 V changing signal at the output:

gain = 5 V / 0.25 V = 20

The amplifier has a voltage gain of about 20. Because volts divide by volts, this simple gain ratio has no unit.

Real tube amplifiers depend on bias, plate voltage, load resistance, tube characteristics, and operating point. For field-style ball-parking, however, the core idea is easy: a gain of 20 means the output signal is roughly twenty times the input signal in the quantity being compared.

Amplifiers can also become oscillators

Once a device can amplify, some of its output can be deliberately fed back to its input. If the amount and phase of that feedback are right, the circuit can continue producing an alternating signal even without an external repeating input. It becomes an oscillator.

That principle would become central to radio transmitters, receivers, test equipment, clocks, musical instruments, and eventually digital electronics. The triode therefore did more than make signals louder; it gave engineers a controllable building block for creating entirely new signals.

Electricity becomes electronics

The path from the light bulb is now clear. Heating a filament led to thermionic emission. Two electrodes made a diode that could rectify. Adding a grid made a triode that could amplify and oscillate.

incandescent lamp → Edison effect → diode → triode
→ amplification → oscillation → electronic circuits

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