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| :''"Dynamic load" redirects here. Not to be confused with [[Dynamic loading]].''
| | The writer is known by the name of Figures Lint. Years in the past we moved to North Dakota. Hiring is her working day job now but she's always needed her personal business. What I adore doing is doing ceramics but I haven't made a dime with it.<br><br>Also visit my blog post :: std testing at home ([http://www.pinaydiaries.com/blog/104745 click the up coming webpage]) |
| An '''active load''' or '''dynamic load''' is a [[Electronic component|component]] or a [[Electronic circuit|circuit]] that functions as a current-stable nonlinear [[resistor]].
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| ==Circuit design==
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| 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]]<ref name=Jaeger>
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| {{cite book
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| |author=Richard C. Jaeger, Travis N. Blalock
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| |title=Microelectronic Circuit Design
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| |year= 2004
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| |edition=Second Edition
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| |publisher=McGraw-Hill Professional
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| |location=New York
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| |isbn=0-07-250503-6
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| |url=http://books.google.com/hi?id=u6vH4Gsrlf0C&pg=RA1-PA1228&dq=%22active+load%22&as_brr=0&sig=9FuiBTRt73yyuZ7MDR9jSDJye_s
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| |page=1228}}
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| </ref> 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 V<sub>CC</sub> on the figures below).
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| ===Common base example===
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| [[Image:Current follower resistor load.svg|thumb|left|160px|Figure 1: Basic NPN common base circuit with resistor load (neglecting [[biasing]] details). Signal is applied at ''V<sub>In</sub>'', output taken from node ''V<sub>out</sub>'' may be a voltage or a current]]
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| [[Image:Common base with active load.svg|right|thumb|160px|Figure 2: Basic NPN common base circuit (neglecting [[biasing]] details). Current source ''I<sub>C</sub>'' represents an active load.]]
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| In Figure 1 the load is a resistor, and the current through the resistor is determined by [[Ohm's law]] as:
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| ::<math>I_C = \frac {V_{CC} - V_{out}} {R_C} </math>.
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| 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 ''V<sub>out</sub>''. which in turn lowers the voltage drop ''V<sub>CB</sub>'' between collector and base, limiting the signal swing at the amplifier output (if the output swing is larger than ''V<sub>CB</sub>'', the transistor is driven out of active mode during part of the signal cycle).
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| In contrast, using the active load of Figure 2, the AC impedance of the ideal current source is infinite regardless of the voltage drop ''V<sub>CC</sub> - V<sub>out</sub>'', which allows even a large value of ''V<sub>CB</sub>''. and consequently a large output signal swing.
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| === Differential amplifiers ===
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| Active loads are frequently used in [[Operational_amplifier#Differential input stage|op-amp differential input stages]], in order to enormously increase the gain.
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| ===Practical limitations===
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| 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.
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| ==Test equipment==
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| In the area of [[electronic test equipment]], an '''active load''' is used for automatic testing of [[electronic power supply|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.
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| One approach to test loads uses a set of [[resistor]]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]] 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]].
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| Just like a resistor, an active load converts the power supply's electrical energy to heat. The heat-dissipating devices (usually [[transistor]]s) in an active load therefore have to be designed to withstand the resulting temperature rise, and are usually cooled by means of [[heatsink]]s.
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| 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.
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| == External links ==
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| == References ==
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| <references/>
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| [[Category:Electronic test equipment]]
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| [[Category:Electronics terminology]]
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The writer is known by the name of Figures Lint. Years in the past we moved to North Dakota. Hiring is her working day job now but she's always needed her personal business. What I adore doing is doing ceramics but I haven't made a dime with it.
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