Hi guys,
Please find the attachment. Here an op-amp is driving a transformer. I am feeding a FFSK signal (audio band) at TP1 and when probing it at various TPs. The following is the signal condition observed in a scope:

• at TP2: distorted and voltage drop.
• at TP3: distorted and voltage drop.
• at TP2 with load (600Ohm) removed: signal wobbling. (could be due to floating).
• at TP3 with load (600Ohm) removed: signal wobbling. (could be due to floating).

What could be the issue.
I need a neat signal (with out distortion) at the load. A level drop is OK.
Can any one help.

Thanks & Regards,
Srikanth.

 

hi,
Set TP1 signal voltage to zero and disconnect the transformer from the OPA output and measure TP2 voltage, it will be Vdd/2.!
What is the DC resistance of the 1:1 transformer primary winding.?

Then re-connect the transformer to the OPA, with TP1 still at zero, re-measure the DC voltage on TP2.

E

 

hi,
Set TP1 signal voltage to zero and disconnect the transformer from the OPA output and measure TP2 voltage, it will be Vdd/2.!
What is the DC resistance of the 1:1 transformer primary winding.?

Then re-connect the transformer to the OPA, with TP1 still at zero, re-measure the DC voltage on TP2.

E

Hi Eric,
Yes the voltage at TP2 when transformer is removed is VDD/2 due to the voltage divider used at the non-inverting terminal of op-amp. And after I connected back the transformer, the voltage at TP2 is still VDD/2.
By the way, I tried matching the transformer input impedance by placing a resistor capacitor series combination ( R=600Ohm , C = 0.1uF). But still the situation is same.

Can we suspect the transformer. The transformer is actually present in another device. I need to interface my hardware (op-amp ) with this device which is said to have a 600Ohm 1:1 transformer at the input.

Can we suspect any think from the transformer point of view...??

 

hi,
Please measure the DC resistance of the transformer windings, let me know.

I have simulated the circuit, using a 1uF and 500R in series with OPA output and transformer primary, it works OK in simulation.
The rt1 input signal is a DialTone wave file.

E

 

You should not apply any DC to the transformer since that will saturate it. You can block the DC by using a capacitor in series with the transformer input.