In previous lectures we explored the triodes and understood their functioning in terms of their use as Signal Amplifiers. We also learned some of their limitations (Miller Effect).

We will explore the same great triode but this time not as an amplifier but as a buffer, but before that, its imperative to first understand what a buffer is and what value a buffer brings on the table.

Einstien had once quoted that “I would spend 90% of the time understanding a problem and only 10% of the time in solving”!

If we go by this then it would make sense to first understand the literal meaning of the term “BUFFER”, perhaps that might lead us to some clues and direction.

In computing a Buffer means, “a temporary memory area in which data is stored while it is being processed or transferred, especially one used while streaming video or downloading audio”, similar meaning exists in more generic uses of the term.

If we extend this use case in audio we may arrive at a logical conclusion closer to the fact. We know that the signals in audio are carried by electrons, in simpler words, it is the flow of current which is carrying the signal, a buffer, in this case may be a mechanism to facilitate the same more efficiently as suggested just by the meaning of the word and we shall soon realize that we are indeed close to the actual stuff.

Electronic Buffer

A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a current buffer) that the load may produce. The signal is ‘buffered from’ load currents. Two main types of buffer exist: the voltage buffer and the current buffer.

A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is infinite and the output resistance zero (output impedance of an ideal voltage source is zero). Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.

If the voltage is transferred unchanged (the voltage gain is 1), the amplifier is a unity gain buffer; also known as a voltage follower because the output voltage follows or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.

These benefits are of tremendous value and in fact any good piece of equipment has at some stage a buffer deployed making these a quintessential element in audio equipment design.

A voltage follower, regardless of the technology used to build it, is a current amplifier. A small available current from the source is usually due to the circuit having a high impedance, so it cannot supply enough current to drive the following circuitry. Most of the time, we are concerned with voltage amplifiers, which (as their name suggests) increase the amplitude of the signal. These are used when the voltage from the source is too low to be useful. In reality, the vast majority of circuits combine both voltage and current amplification, although the latter is often not the primary goal. It comes ‘free’ with the circuit (especially opamps).

These days when a voltage follower is needed, it will almost always be an opamp connected as a unity gain amplifier. When using a vacuum tube the common name of such a buffer is the “CATHODE FOLLOWER”.

For a detailed Technical Description, refer the Wireless World Magazine article published in 1945.

Typical Use Cases for a Buffer

1. Preamplifier: for supply current though longer cable runs without impacting tonality and fidelity
2. Guitar Effects Chain
3. Microphones
4. Start and end of long series interconnected equipments
5. Headphone amplifiers