An active or dynamic load is a component or a circuit behaving as a current-stable nonlinear resistor. This term may refer to a component of circuit design, or to a type of test equipment.

## Circuit design

In circuit design, an active load is a circuit component made up of active devices, such as transistors, intended to present a high small-signal 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. Most commonly the active load is the output part of a current mirror[1] 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

Figure 1: Basic NPN common base circuit with resistor load (neglecting biasing details). Signal is applied at VIn, output taken from node Vout may be a voltage or a current
Figure 2: Basic NPN common base circuit (neglecting biasing details). Current source IC represents an active load.

In Figure 1 the load is a resistor, and the current through the resistor is determined by Ohm's law as:

$I_C = \frac {V_{CC} - V_{out}} {R_C}$.

As a consequence of this relation, the voltage drop across the resistor is tied to the current at the Q-point. If the bias current is fixed for some performance reason, any increase in load resistance automatically leads to a lower voltage for Vout. 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 - Vout, 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, 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, an active load is used for automatic testing of power supplies 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 resistors 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 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.

Just like a resistor, an active load converts the power supply's electrical energy to heat. The heat-dissipating devices (usually transistors) in an active load therefore have to be designed to withstand the resulting temperature rise, and are usually cooled by means of heatsinks.

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.

Wikimedia Foundation. 2010.

Нужно сделать НИР?

### Look at other dictionaries:

• active load — aktyvioji apkrova statusas T sritis fizika atitikmenys: angl. active load; ohmic load; resistive load vok. ohmsche Belastung, f; Widerstandsbelastung, f; Wirklast, f rus. активная нагрузка, f; омическая нагрузка, f pranc. charge active, f; charge …   Fizikos terminų žodynas

• active load — aktyvioji apkrova statusas T sritis automatika atitikmenys: angl. active load; real load vok. Widerstandsbelastung, f; Wirkbelastung, f; Wirklast, f rus. активная нагрузка, f pranc. charge active, f; charge ohmique, f; charge résistive, f …   Automatikos terminų žodynas

• Active learning — is an umbrella term that refers to several models of instruction that focus the responsibility of learning on learners. Bonwell and Eison (1991) popularized this approach to instruction. This buzz word of the 1980s became their 1990s report to… …   Wikipedia

• Load balancing (computing) — Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization,… …   Wikipedia

• Active rectification — is a technique for improving efficiency of Diode Bridge rectifiers. It consists of replacing a diode with a transistor (usually a power MOSFET). Applications In Full Bridge Rectifiers, the voltage drop of a diode (typically around 0.7V for a… …   Wikipedia

• Active Body Control — Active Body Control, or ABC, is the Mercedes Benz brand name used to describe fully active suspension, that allows control of the vehicle body motions and therefore virtually eliminates body roll in many driving situations including cornering,… …   Wikipedia

• Active dynamic filtering — Active dynamic filtering, ADF, is a three phase power electronics based power flow control technology that can effectively be used to isolate and remove electrical behaviours that cause excess losses and disruptions in the electrical… …   Wikipedia

• Active power conditioner — (active power filters) refers to power electronic devices that are used in industry or commercial facilities to improve electrical power quality and remove behaviours that cause losses and disruptions, such as harmonics, reactive power, flicker… …   Wikipedia

• Active Valve Control System — (AVCS) is an automobile variable valve timing technology used by Subaru. It varies the timing of the intake valves by using hydraulic oil pressure to rotate the camshaft in order to provide optimal air flow in and out of the engine. The system is …   Wikipedia

• Active Fuel Management — See the main article on variable displacement for other similar systems Active Fuel Management (formerly known as Displacement on Demand) is a trademarked name for the automobile variable displacement technology from General Motors. It allows a… …   Wikipedia