# Differential amplifier with DC offset

• posted

Hi, I made a differential amplifier using JFETs. The output of this amplifier sits at 5 volt DC offset. Now before i feed it to another stage, i want to get rid of the DC component. I want to avoid using blocking capacitors. I was planning to use a pnp differential stage to level shift, but have no idea how to bias the input matched PNPs so that they can take small signal with a 5 volt DC offset. Please help, i have tried using spice but just can't get it working.

• posted

snipped-for-privacy@gmail.com schrieb:

Use a constant current source from the FETs common source point to the negative supply( hope it exists ). Each side will drive half the current of the current source (Icc) with delta_Vin=0. The drain resistors should have a value of R= V+/Icc. This leads to a voltage drop across the drain resistors of about Vcc, so you'll get an output voltage of (nearly) 0V.

Hope it helps - Udo

• posted

Udo Piechottka schrieb:

Sorry, R=V+/(Icc/2)

• posted

thanks Udo, I understand that part. I think i didn't explain my setup properly. I have a first stage differential amplifier using JFETs which gives output with a 5 volt offset. Now i want to amplify this signal more, i can't feed it to the intrumentation amplifier without blocking capacitors. For this reason i was thinking to design another differential stage using PNP BJTs to amplify as well as DC level shift the signal to 0 volts. Then if needed i can feed this to a instrumentation amplifier. So currently, i am trying to design a PNP differential stage which can take 5 volts as the base and collector around 0 volts.

• posted

snipped-for-privacy@gmail.com schrieb:

The problem stays the same.

You'll raise the total error with each additional stage.

So if you are planning to use an instrumentation amplifier the amp will glad to see the differential voltage from your first stage. To increase the gain select the appropriate amp.

What about the power supply? Please desribe what you are intended to realize...(schematics?)

Udo

• posted

Is it a constant offset though or does it vary with temperature etc

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