Does anybody have some ideas on how to design a voltage buffer which can cover VSS? the input range is from VSS to VDD/2.

of course, some sort of level shifter based structures can do the job. but are there any structures which have no level shifting? Due to the application, i want the buffer only to change the impedance, but not the voltage level.

thanks.

raja.cedt wrote on May 26^th, 2009, 2:28am:


sorry i didn't formulate the question very clearly. the following stage of the buffer has a quite low input impedance, so i use the buffer only as a driver, but i want the dc level of the buffer input and output to be the same.

how will the pmos opamp work? i assume you mean a noninverting configuration of the opamp. that means both inputs and output of the opamp have to go down to VSS. but the output level cannot be lower than one Vdsat above VSS.

solidstate wrote on May 27^th, 2009, 5:56am:


the signal source is really going down to vss, and a dc shift is also sort of buffer.

i think i asked a stupid question in the very first place. 'such circuit is impossible' is also a good answer to know

Just an idea:
By having a resistive load you could operate the buffer lower than vdsat.

Cheers

Consider just a simple differential pair with PMOS input transistors and resistors on the load side (or even only a source-follower) in unity-gain feedback configuration. This circuit should still work at relatively low input voltages.

Regards