Active Load
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An active load or dynamic load is a
component Circuit Component may refer to: •Are devices that perform functions when they are connected in a circuit.   In engineering, science, and technology Generic systems * System components, an entity with discrete structure, such as an assem ...
or a circuit that functions as a current-stable nonlinear
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
.


Circuit design

In circuit design, an active load is a circuit component made up of ''active devices'', such as
transistors upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
, intended to present a high
small-signal Small-signal modeling is a common analysis technique in electronics engineering used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC ...
impedance yet not requiring a large DC voltage drop, as would occur if a large resistor were used instead. Such large AC load impedances may be desirable, for example, to increase the AC gain of some types of
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
. Most commonly the active load is the output part of a
current mirror A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...
and is represented in an idealized manner as a current source. Usually, it is only a ''constant-current resistor'' that is a part of the whole current source including a ''constant voltage source'' as well (the power supply ''VCC'' on the figures below).


Common base example

In Figure 1 the load is a resistor, and the current through the resistor is determined by
Ohm's law Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equat ...
as: :I_C = \frac {R_C}. As a consequence of this relation, the voltage drop across the resistor is tied to the current at the
Q-point In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processin ...
. If the bias current is fixed for some performance reason, any increase in load resistance automatically leads to a lower voltage for ''V''out. which in turn lowers the voltage drop ''VCB'' between collector and base, limiting the signal swing at the amplifier output (if the output swing is larger than ''VCB'', the transistor is driven out of active mode during part of the signal cycle). In contrast, using the active load of Figure 2, the AC impedance of the ideal current source is infinite regardless of the voltage drop ''VCC'' − ''V''out, which allows even a large value of ''VCB''. and consequently a large output signal swing.


Differential amplifiers

Active loads are frequently used in op-amp differential input stages, in order to enormously increase the gain.


Practical limitations

In practice the ideal current source is replaced by a
current mirror A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...
, which is less ideal in two ways. First, its AC resistance is large, but not infinite. Second, the mirror requires a small voltage drop to maintain operation (to keep the output transistors of the mirror in active mode). As a result, the current mirror does limit the allowable output voltage swing, but this limitation is much less than for a resistor, and also does not depend upon the choice of bias current, leaving more flexibility than a resistor in designing the circuit.


Test equipment

In the area of
electronic test equipment Electronic test equipment is used to create signals and capture responses from electronic devices under test (DUTs). In this way, the proper operation of the DUT can be proven or faults in the device can be traced. Use of electronic test equipmen ...
, an active load is used for automatic testing of
power supplies A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a res ...
and other sources of electrical power to ensure that their output voltage and current are within their specifications over a range of load conditions, from no load to maximum load. One approach to test loads uses a set of
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s of different values, and manual intervention. In contrast, an active load presents to the source a resistance value varied by electronic control, either by an analogue adjusting device such as a multi-turn
potentiometer A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrume ...
or, in automated test setups, by a digital computer. The load resistance can often be varied rapidly in order to test the power supply's
transient response In electrical engineering and mechanical engineering, a transient response is the response of a system to a change from an equilibrium or a steady state. The transient response is not necessarily tied to abrupt events but to any event that affec ...
. Just like a resistor, an active load converts the power supply's electrical energy to heat. The heat-dissipating devices (usually
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s) in an active load therefore have to be designed to withstand the resulting temperature rise, and are usually cooled by means of
heatsink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, the ...
s. For added convenience, active loads often include circuitry to measure the current and voltage delivered to the inputs, and may display these measurements on numeric readouts.


References

Electronic test equipment Electrical components