Signals
An audio signal can be transmitted either as a voltage or a current. The construction of the universe is such that almost always the voltage mode is more convenient; consider for a moment an output driving more than one input. Connecting a series of high-impedance inputs to a low-impedance output is simply a matter of connecting them in parallel, and if the ratio of the output and input impedances is high there will be negligible variations in level. To drive multiple inputs with a current output it is necessary to have a series of floating current-sensor circuits that can be connected in series. This can be done [1], as pretty much anything in electronics can be done, but it requires a lot of hardware, and probably introduces performance compromises. The voltage-mode connection is just a matter of wiring.
Obviously, if there’s a current, there’s a voltage, and vice versa. You can’t have one without the other. The distinction is in the output impedance of the transmitting end (low for voltage-mode, high for current-mode) and in what the receiving end responds to. Typically, but not necessarily, a voltage input has a high impedance; if its input impedance was only 600 Ω, as used to be the case in very old audio distribution systems, it is still responding to voltage, with the current it draws doing so a side issue, so it is still a voltage amplifier. In the same way, a current input typically, but not necessarily, has a very low input impedance. Current outputs can also present problems when they are not connected to anything. With no terminating impedance, the voltage at the output will be very high, and probably clipping heavily; the distortion is likely to crosstalk into adjacent circuitry. An open-circuit voltage output has no analogous problem.
Current-mode connections are not common. One example is the Krell Current Audio Signal Transmission (CAST) technology which uses current-mode to interconnect units in the Krell product range. While it is not exactly audio, the 4–20 mA current loop format is widely used in instrumentation. The current-mode operation means that voltage drops over long cable runs are ignored, and the zero offset of the current (i.e. 4 mA zero) makes cable failure easy to detect – if the current is zero, you have a broken cable.
The old DIN interconnection standard was a form of current-mode connection in that it had voltage output via a high output impedance, of 100 kΩ or more. The idea was presumably that you could scale the output to a convenient voltage by selecting a suitable input impedance.
The drawback was that the high output impedance made the amount of power transferred very small, leading to a poor signal-to-noise ratio. The concept is now wholly obsolete.
Amplifiers
At the most basic level, there are four kinds of amplifier, because there are two kinds of signal (voltage and current) and two types of port (input and output). The handy word ‘port’ glosses over whether the input or output is differential or single ended. Amplifiers with differential input are very common – such as all opamps and most power amps – but differential outputs are rare and normally confined to specialised telecoms chips.
To summarise the four kinds of amplifier:
TABLE 1.1 The four types of amplifier | Amplifier type | Input | Output | Application |
| Voltage amplifier | Voltage | Voltage | General amplification |
| Transconductance amplifier | Voltage | Current | Voltage control of gain |
| Current amplifier | Current | Current | ? |
| Transimpedance amplifier | Current | Voltage | Summing amplifiers, DAC interfacing |
Voltage amplifiers
These are the vast majority of amplifiers. They take a voltage input at a high impedance and yield a voltage output at a low impedance. All conventional opamps are voltage amplifiers in themselves, but they can be made to perform as any of four kinds of amplifier by suitable feedback connections. Figure 1.1 a shows a high-gain voltage amplifier with series voltage feedback. The closed-loop gain is (R1 R2)/R2.
Transconductance amplifiers
The name simply means that a voltage input (usually differential) is converted to a current output. It has a transfer ratio A = Iout/Vin, which has dimensions of I/V or conductance, so it is referred to as a transconductance amplifier. It is possible to make a very simple, though not very linear, voltage-controlled amplifier with transconductance technology – differential-input operational transconductance amplifier (OTA) ICs have an extra pin that give...
