Hi ,Sir
when AD8031 is used as a buffer, the AC curve is correct.
below picture is the simulation result
but when the op. is used as a line driver, the ac curve is un-normal.
the expected value of vout is 6db, but the result is uncorrect.
what should i do to solve the issue?
Hello Dyingh,
as a generell comment: if you have a problem with a simulation file, it is always a good idea to add the .asc file. Then others can easily work on the problem and find solutions.
As for the specific problem: the AC simulation takes place around a DC operating point. The DC operating point in your simulation is selected so that Vout is approximately 0V. More precisely: Vout divided by 2 (voltage divider R2 - R1) will correspond to the offset voltage of the OPV.
Depending on the random sign of the offset voltage, this can either mean that the npn transistor is slightly conducting (base voltage at approx. 0.6V) or that the npn transistor is completely blocked (base voltage close to Vee).
In your case, the npn transistor blocks. At this operating point, the small signal excitation of the AC analysis does not cause the npn to conduct - the base of the npn sees only "a small signal around -5V".
You will get the expected result if you set the operating point so that the transistor conducts. To do this, the DC voltage of the source must be safely above the offset voltage of the OPV.
The attached simu was carried out once with a DC input voltage of U_DC=0V (npn blocks, AC-gain of the circuit is -75dB), once with U_DC=100mV (npn conducts, the AC gain of the circuit is 6dB).
You may also try a short transient simulation to check the different DC operating points.
best regards
Achim
