Odd an amplifier that need branch it is integrated circuit, include an operation amplifier (carry is put) and not less than 4 are used identical enclosed accurate resistor. Change into sheet to carry signal to be restrained at the same time to needing to distinguish difference signal in all the imitate design personnel of modular signal, they are very useful compose build piece. For example, the INA134 intent that the graph shows 1 times is to use as the circuit receiver that suits interface of the frequency that need branch.

Although great majority designs personnel to feel this kind of simple component part piece use very relaxed and satisfied, but the author or discovery are using them from time to tome a respect often is ignored: The two inputs end of the amplifier that need branch has different effective input resistance. Author place says ” effective input resistor ” the input resistor that those who point to is the moving generation that by in-house resistor block is worth and carry puts.

Graph the 2 typical configuration that showed INA134, the input voltage that has number and electric current and in-house carry put the voltage that inputs node place.

 Input end to every, equational 1 all be explanation of effective input resistor:

Let us part from easier ministry first only then: Input end in phase. See a picture 2 medium sketch map, you can discover R3 and R4 are series connection. Be entered without electric current suppose or leave carry to put an input to carry, so equational the effective input resistor that 2 computation give is only:

Now, let our focusing input end instead. Think back to good fortune puts regulation: The two inputs end that carry puts should be in identical electromotive force from beginning to end (equational 3) :

You still can see R3 and R4 were inputting end to form a voltage divider in phase. The movement that 4 computation give equation puts inphase input to carry (VP) voltage is:

This why very important? Oh, because this voltage can decide partly,input the effective input resistor of end instead. Considering the electricity that carries R1 (IIN (N) ) be equal to the voltage that crosses R1 to divide with its resistor (equational 5) :

Return equation 1, with equation 5 replace input electric current, you can reach equational 6 — calculable the general equation that goes out to input resistor instead:

Since you know to input the voltage of end instead (VN) the voltage that will be equal to inphase input to carry (VP) , you can use equation 4 replace equation 6 medium VN, reach equational 7:

Attention, the effective input resistor that inputs end instead depends on actually two rate that input upright voltage. Want to understand this how to can affect your application, consider a case please: In the INA134 that frequency circuitry uses in receiver application, among them two voltage amplitude that input end are equal but polarity is contrary (equational 8) :

Had turned round to treat form of one lower part 2, the effective input resistor that inputs end in phase is quite simple:

Nevertheless, two input those who carry voltage to turn over relevant department to be able to input the effective input resistor of end to produce major effect to turning over (equational 9) :

The effective input resistor that inputs end instead is inphase input end 1/3 what input resistor effectively. Accordingly, when choosing to input coupling capacitor and filter wave circuit, you must consider the impedance that inputs end instead is inferior. In addition, the amplifier that inputs of any amplifier of drive moment cent carry must can drive inputs the impedance with inferior end instead.

Use simple circuit, often be the basiccest side that its run a trouble can be caused in the lab. Accordingly, do not ignore the thing of clearly please!

Origin: Heart state apparatus