Although the voltaic lens such as current source of such as Howland and circuit are when education,attribute imitate circuit part, have as before quite the engineer of one part is in the apt when defining accurate imitate circuit to output considers an issue from the angle of voltage. This very regrettablly, because voltaic output can offer an advantage in many sided, the signal of road of annulus of imitate electric current that incorporates loud noise environment to fall (0 MA comes to 20 MA and 4 MA 20 MA) , and in do not undertake in the light of bigger voltage imitate signal n is changed below the circumstance of optics of have the aid of or magnetic segregation technology. The article summed up one part to have a technology, supply many practical circuit.

 

Getting stable electric current output is extremely simple thing, the simplest method uses voltaic lens namely: —— uses two identical transistor to be made with a chip, thereby completely consistent —— is like craft, dimension and temperature the graph is shown 1 times be linked together. The base of two parts of an apparatus – shoot extremely voltage is identical, because this flows into the output of collector T2,electric current is equal to the input electric current that flows into collector T1.

 

Graph 1. Lens of basic electric current
 

T1 of this analysis hypothesis and T2 are identical and isothermal, and their voltaic gain is extremely tall, so that but oversight base electric current. It returns meeting oversight inchoate voltage, make collector electric current is changed along with collector voltage change.

 

Can use NPN or PNP transistor to comprise these voltaic lens. N transistor paralell connection comprises T2, the N that outputs electric current to be input electric current times, if pursue,2a place is shown. Be like T1 by M transistor composition, t2 by N transistor composition, the N/m that exporting electricity will be input electric current times, if pursue,2b place is shown.

 

Graph 2. (Lens of A) multistage electric current (B) is not lens of integral scale electric current to be able to rise union of 3 T2 collector, get 3IIN
 

If inchoate voltage impact is very big, can use slightly lens of complex Wilson electric current falls to lowest. 3 transistor and 4 transistor version are shown 3 times like the graph. 4 transistor version is more accurate, and have wider dynamic range.

 

Graph 3. T4 of lens of Wilson electric current is optional parts of an apparatus, but use it ameliorable precision and dynamic limits
 

Need is crossed guide amplifier (when Voltage_in/current_out) , can use an amplifier of operation of an only power source, BJT or FET (MOSFET is optimal choice normally, because it is nonexistent error of base electric current) and a definition is crossed guide the accurate resistance of the value will comprise, if pursue,4 are shown.

 

Graph 4. Cross guide amplifier VIN–IOUT
 

This circuit simple, price is not high. The voltage on MOSFET grid can install the electric current in MOSFET and R1, make the voltage V1 on R1 is equal to input voltage VIN.

 

If need to use voltaic lens in odd chip IC, had better use lens of simple transistor electric current. However, if use schism type circuit, the price that its match resistor to hold high (the price is high because demand is finite,be, and rather than creates difficulty) will make pursue lens of 5 medium operation amplifier electric current make the cheapest technology. Lens of this electric current by cross guide amplifier and an extra resistance are comprised.

 

Graph 5. Lens of operation amplifier electric current
 

Voltaic lens is had opposite taller, input impedance nonlinearly sometimes, accordingly they must by tall impedance current source (also call rigid current source sometimes) provide electricity. If input electric current to must have low impedance to attract voltaic faculty, need to use operation amplifier. The graph is shown 6 times for lens of electric current of two low ZIN.

 

Graph 6. (Lens of electric current of A) low ZIN is turned over (lens of electric current of B) low ZIN is inphase
 

Use lens of basic electric current and current source, be inputted and polarity of output electric current is identical. Normally, the ejaculation that outputs transistor extremely / source pole direct or through detecting resistor ground connection, and output electric current from collector / leakage pole flows into load, direct current source of other terminal join. Such doing be not always is very convenient, require ground contact when a laden terminal especially when. If the graph is shown 7 times, if circuit uses the ejaculation of its direct current source extremely / source pole comes compose is built, nonexistent this problem.

 

Graph 7. Lens of ground connection load current
 

If electric current or voltage input referenced ground, must use n changeover. Many circuit can come true; And graph 8 medium systems are applicable below a lot of circumstances. This simple circuit uses the voltaic lens on direct current source of drive of ground connection current source, thereby drive is laden. Attention, voltaic lens may have gain, because this signal electric current does not need,resemble load current so tall.

 

Graph 8. N changes voltaic lens
 

At present till, the —— electric current that the circuit that we discuss is odd polarity flows on a direction —— but bipolar voltaic circuit also is feasible. The simplest, use the most extensive should count pump of Howland electric current, if pursue,9 are shown. This simple circuit has a lot of problems: The precision demand that it matchs to resistor is extremely high, in order to obtain high output impedance; Input source impedance can increase R1 resistance, because of its numerical value must very low reduce with the oldest rate match an error; Power source tension must be gotten higher than voltage of the biggest output much; And the CMRR function of operation amplifier must relatively good.

 

Graph 9. Pump of Howland electric current is bipolar voltaic output
 

Now, price of high-powered appearance amplifier is not high, because this uses an operation amplifier, an appearance amplifier and an electric current to detect,resistor is comprised bipolar current source is very convenient, if pursue,10 are shown. This kind of circuit wants than pump of Howland electric current more simple, do not rely on resistor network (besides compositive appearance amplifier the sort of) , and voltage amplitude is in the 500 MV less than of every power source.

 

Graph 10. Bipolar voltaic operation amplifier
 

At present till, the circuit that we discuss is the amplifier that has output of accurate electric current. Of course, they can be used together with fixed input, provide accurate current source, but compose proposes a simpler double end current source also is feasible. Small electric current is fiducial voltage source ADR291 has what 10 μA controls to need machine electricity, typical temperature coefficient is 20nA/°C. If the graph is shown 11 times, after joining laden resistance, criterion 3 V is to the fiducial electric current inside limits of 15 V power source (2.5/R + 0.01) MA, among them R is laden resistor, the unit is KΩ .

 

Graph 11. Double end current source
 

If precision is not a problem, and ask tigidity is odd only polarity current source, can use extinct model JFET and current source of composition of a resistor. If the graph is shown 12 times, this kind of configuration is when temperature happening change not very stable, and to given R value, the electric current of each parts of an apparatus may have quite big difference, but this configuration is simple and cheap.

 

Graph 12. JFET current source
 

Recently, I need to design power source for certain LED. The meeting when friend of a few engineers thinks I offer LED to undertake moving smooth volatile current source in the design encounters a few difficulty. In fact, I just changed my costume or dress simply the power source of ” of “ black brick of notebook computer (spend a few cent to be bought from flea market) do decided. Graph 13 those who show is the power source circuit that the course revises simply, can offer for LED constant electric current. Use little output electricity, it can secure output voltage to work normally.

 

Graph 13. Revise power source of black brick switch to be outputted in order to offer current limliting
 

To get alterable electricity, the potentiometer that —— of will fiducial voltage comes from black brick or this locality —— brings to bear on to be represented at P1 and P2 place. OPA2 and MOSFET output small electric current through R1, on its generation pressing falls. Load current has flowed to detect resistor. If detect,the voltage on resistor is pressed as a result of what load current exceeds R1 to go up fall and drop somewhat, so OPA1 output will rise, enclothe the voltage control in the brick, restrict its to output voltage, prevent to output electric current to be restricted to be worth more than.