Use of buffer at the output of amplifier

[Rahul Sharma]

Why do we use buffer after amplifier stage? In other word tell me the use / application of buffer in analog circuits.

 

[KlausST]

Hi

this is a very vague question.

If you need good answers you should first give good informations.
This can be:
* example circuits (as picture)
* documents (as link or attached)
* part names
* detailed descriptions

****
I only can give a common answer:
--> Usually one uses a buffer after an amplifier to increase output current.

But I´m not sure if this applies to your problem.

Klaus

 

[surianova]

hi

Buffer will isolate the output loading from your circuit. The loading can be resistive or capacitive loading.
Buffer provide high impedance at the input and low impedance at the output. The maximum gain of the buffer is 1.

 

[Rahul Sharma]

- - - Updated - - -

hi

Buffer will isolate the output loading from your circuit. The loading can be resistive or capacitive loading.
Buffer provide high impedance at the input and low impedance at the output. The maximum gain of the buffer is 1.

If i want to understand it in terms of output current which is depending of load. Such as one condition when load is small say 8 ohm or so and another condition when load is very high. What will be the possible explaining on using of buffer in terms of output current.

 

[KlausST]

Hi,

As said before: you need to give more informations.

Now the only value you give is "8 Ohm".
With this single value you can't calculate anything.

To be able to calculate the current you need "voltage" and Ohm's law.

R = U / I. --> I = U / R

I1 = U / 8Ohm
I2 = U / "very high" Ohm

Klaus

 

[surianova]

buffer will provide impedance of 1/gm and in parallel with the load, RL. The gain of the buffer is gm(1/gm// RL).

In order to get gain =1, the output impedance of the buffer which is 1/gm must be 10 times smaller than RL. So, small smaller RL, you need even smaller 1/gm. Gm need to increase to get smaller 1/gm. Gm determine by the current. Higher gm needs higher current. That explain why buffer current depends on RL or load current.

 

[LvW]

buffer will provide impedance of 1/gm and in parallel with the load, RL. The gain of the buffer is gm(1/gm// RL).

To avoid misunderstandings: This answer covers single-transistor buffers only (common collector configuration).