Need a Buffer circuit

[hithesh123]

I need a discrete buffer (not opamp).
I tried Emitter follower. I can't live with the 0.7v drop at the output. I tried JFET buffer. The output resistance is higher than what I want.
The output resistance needs to be less than 100 ohms.

The input to the buffer is coming from a voltage divider(6K).
Any ideas?

 

[fareen]

why not use common base or common gate configuration they are current buffers.
it accept the input signal at a low input resistance and delivers almost equal current at the output.
while common collector can be used as voltage buffer,for connecting a high resistance source to a low resistance load and as the output stage.

 

[hithesh123]

Fareen, you mean something like this (attached).
https://obrazki.elektroda.pl/98_1307881819.png
The output is connected to a line receiver(rs422). This ckt creates offset at the receiver inputs.
How do you pick the coupling capacitors?

 

[LvW]

I need a discrete buffer (not opamp).
I tried Emitter follower. I can't live with the 0.7v drop at the output.

Does this mean that no ac coupling is allowed? Otherwise it would be no problem to use a common collector stage with a dc output of zero volts (and two appropriate supply voltages).
And -as far as FET's are concerned - why is it not possible to achieve an output resistance below 100 ohms?

 

[hithesh123]

Does this mean that no ac coupling is allowed? Otherwise it would be no problem to use a common collector stage with a dc output of zero volts (and two appropriate supply voltages).
And -as far as FET's are concerned - why is it not possible to achieve an output resistance below 100 ohms?

Can you point me FET buffer to a ckt. I tried earlier today. The input stage of diff rcr is a strange beast - page 3 in AM26LS32 datasheet http://ronja.twibright.com/datasheets/line_receivers_driver/am26ls32a.pdf

 

[fareen]

yeah i was referring a circuit sumthing like that.wee pick the coupling capacitor,keeping in view the range of band width required and lower cuttoff frequency.these coupling capacitor decide the lower frequency response

 

[FvM]

I don't understand your reference to AM26LS32, in my opinion, there's no reasonable prototype of an analog buffer circuit that can be copied from the datsheet.

Your specifications aren't completely clear, I think. Your requesting Ro < 100 ohm and Vos < 0.7 V. How much offset and offset drift is acceptable? How about the signal range? Even a perfectly matched discrete buffer will have some offset and a temperature drift of the same. A npn + pnp follower pair is a very simple circuit with reduced voltage shift. If low gain error is an objective, you would possibly go for a feedback buffer structure. There are many options, you are mainly limited my your power of imagination.

P.S.: Page 3 of the datasheet is showing an input clamping circuit, not a buffer, i think.

 

[hithesh123]

FvM,

I need something close to an opamp buffer. High input resistance (something like 100K to 1M) and low output resistance.
The problem I have now, is that if I connect the output of voltage divider directly to the line receiver. The line receiver introduces an offset of about 200mV.
I need to eliminate this. The 5K, 1K voltage divider output is connected to diff line rcr. I get an offset at the input of diff rcr.

 

[FvM]

Apparently I still don't understand the application problem. A "line receiver" is a digital circuit in my understanding. Where do you need an analog buffer in this situation?

 

[hithesh123]

Apparently I still don't understand the application problem. A "line receiver" is a digital circuit in my understanding. Where do you need an analog buffer in this situation?

Well, true. But its getting attenuated by 6x. So 5V becomes 0.8V
That's why I need a buffer.
the 0.8v is not a problem for the line receiver, it's sensitive down to 0.2V.