can anyone explain this about op amp ?

[desperado1]

while drawing the equivalent circuit of opamp we see that there is no electrical connection between vin and the output circuit. then how it is possible that the output is gain times the input signal. By inspection it looks like an open circuit. This is taking the brain out of me. please help me..................

 

[subharpe]

No, there is no electrical connection between the input and the output. But the OPAMP is a voltage amplifier which is nothing but a voltage dependant voltage source. If you see the output section there is a volgage amplifier which is A*vin. This inherently means there is some electrical connection which we don't need to bother about, but it is always there.

For example, in low freq. BJT equivalent circuit also you will find the same problem, as there is no electrical connection between the input and output and there is a current amplifier hfe * ib. This also is the same.

 

[xulfee]

it is a voltage amplifier,its equation may be Vo=A*Vin,
where A is the gain or ratio which amplifies the input. if it is in open loop connection gain may be 10^6.in closed loop case we adjust gain by combination of resistance

 

[vsnute]

Proportionality between Vin nad Vout wouldn't be possible without a connection. The equivalent circuit just doesn't make it evident, but the voltage controlled voltage genarator (A*Vin) means some kind of electrical connection exists.

 

[LvW]

Remember the equivalent circuit for a BJT which also shows explcitely no electrical connection between input and output. But it incorporates a current source controlled by the input signal.

 

[desperado1]

Remember the equivalent circuit for a BJT which also shows explcitely no electrical connection between input and output. But it incorporates a current source controlled by the input signal.

Can you explain me in more detail please since there seems no electrical connection in this case ?

 

[FvM]

Please consider that an equivalent circuit isn't physically real. In a real device, there is either a current or a voltage carrying the signal.

 

[LvW]

Can you explain me in more detail please since there seems no electrical connection in this case ?

It is the principle of an equivalent circuit of a real device (BJT or opamp or something else) to model the real input-output behaviour of this device using as less parts/units as possible. In this context, controlled sources play an important role.
These are artificial elements which establish an input-output relationship simply by a mathematical formula without the necessity to show visually a wire connection between input and output. Is the principle clear now ?

 

[xulfee]

u may also consider that input n output r not directly connected,look at schematic diagram of opamp,may it help u to understand how input & output r actully linked

 

[hrazadi]

Obviously there is a connection between input and output!!! But in the modeling of Op Amps the ratio between input and output voltages is ideally infinite. Because of the nature of Op Amps, Vin+, Vin- and Vout are in a circuit that produces ideally that relation.