The analysis of circuit of source of 6 kinds of the most commonly used constant current and quite
[introduction] constant current circuit has a lot of circumstances to need a circumstance to output impedance to be the constant current source of 0 not only, also need to input impedance to be the constant current source of infinity, it is small below the circuit of constant current of a few kinds of odd polarity that writes summary:
Type 1:
Feature: Use carry is put, high accuracy
Export electricity: Iout=Vref/Rs
Type 2: Feature: Use shunt-wound manostat, simple and high accuracy exports electricity: Iout=Vref/Rs
Detect voltage: Differ according to Vref (1.25V or 2.5V)
Type 3:
Feature: Use transistor, simple, low precision
Export electricity: Iout=Vbe/Rs
Detect voltage: Make an appointment with 0.6V
Type 4: Feature: Reduce a kind the temperature of the Vbe of 3 changes, low, medium precision, low voltage detects output electric current: Iout=Vref/Rs
Detect voltage: Make an appointment with 0.1V ～ 0.6V
Type 5:
Feature: Use JEFT, exceed low noise
Export electricity: By JEFT decision
Detect voltage: Concern with JEFT
Among them type 1 for basic circuit, when the job, input voltage Vref and proportionate of output electric current detect voltage Vs(Vs=Rs×Iout) is equal, if the graph is shown 5 times
Note: Is=IB+Iout=Iout(1+1/hFE) among them 1/hFE is like output level to use transistor for the error heft of base electric current can arise when electric current detects this one error, do not allow when this kind of circumstance when, can adopt a plan 6 are shown use FET pipe in that way
Is=Iout-IG
Type 2, this is to use carry to put with Vref (2.5V) the shunt-wound manostat circuit of unifinication, because the Vref of this kind of circuit is as high as 2.5V, so power source uses range narrower type 3, this is the circuit that replaces carry to put with transistor, because use the Vbe of transistor (make an appointment with 0.6V) the circuit that replaces Vref, accordingly, the temperature change of Vbe is without change ground to show in be being outputted now, thereby the precision type that is less than expectation 4, this is to use the circuit that follows temperature change to be in charge of compensation Vbe, because detect,voltage also is controlled under 0.1V, should this, power source uses range very wide type 5, this is the circuit that uses J-FET, change Rgs can make output electric current to achieve IDSS of electric current of leakage pole saturation, because noise is very little also, accordingly, this kind of circuit is used to also have fair value when noise becomes an issue, do not receive RGS in this circuit, criterion voltaic value becomes IDSS, such, j-FET received diode form to turn diode of “ constant current into ”
Above circuit is electric current is absorbed model circuit, but besides the type 2 beyond, if change Vref polarity and use semiconductor cell, can become electric current to spit model circuit.