Class B amplifier cross-over distortion problem.

[[email protected]]

Hi! I was simulating a class B amplifier to see the distortion. At
first I did not include a load resistor. The simulation result was no
distortion at the output and the output signal is the same as the
input. After that, I added a 1k load resistor. There was distortion but
the output signal amplitude is only 10nV.As I increased the value of
the load resistor, the distortion reduced and the amplitude increased.
I wondered why this was happening. Can someone tell me why the load
resistance affected the distortion? Thanks.


Amy

* Schematics Netlist *

V_V2 0 $N_0001 5v
V_V3 $N_0002 0
+SIN 0 1m 1k 0 0 0
V_V1 $N_0003 0 5v
M_M16 $N_0003 $N_0002 $N_0004 $N_0004 MbreakN
M_M17 $N_0001 $N_0002 $N_0004 $N_0004 MbreakP
R_R1 0 $N_0004 1k

 

[Andrew Holme]

Hi! I was simulating a class B amplifier to see the distortion. At
first I did not include a load resistor. The simulation result was no
distortion at the output and the output signal is the same as the
input. After that, I added a 1k load resistor. There was distortion but
the output signal amplitude is only 10nV.As I increased the value of
the load resistor, the distortion reduced and the amplitude increased.
I wondered why this was happening. Can someone tell me why the load
resistance affected the distortion? Thanks.


Amy

* Schematics Netlist *

V_V2 0 $N_0001 5v
V_V3 $N_0002 0
+SIN 0 1m 1k 0 0 0
V_V1 $N_0003 0 5v
M_M16 $N_0003 $N_0002 $N_0004 $N_0004 MbreakN
M_M17 $N_0001 $N_0002 $N_0004 $N_0004 MbreakP
R_R1 0 $N_0004 1k

Is that a 1mV input amplitude? Try a much bigger input signal. It needs to
exceed the threshold voltage, to turn the MOSFETs on.

 

[Eeyore]

Hi! I was simulating a class B amplifier to see the distortion. At
first I did not include a load resistor. The simulation result was no
distortion at the output and the output signal is the same as the
input. After that, I added a 1k load resistor. There was distortion but
the output signal amplitude is only 10nV.As I increased the value of
the load resistor, the distortion reduced and the amplitude increased.
I wondered why this was happening. Can someone tell me why the load
resistance affected the distortion? Thanks.

You'll need to understand basic circuit theory rather than expect Spice to give
you the answer to that.

Did you not learn that Class B will always in practice produce significant
distortion ?

The reason btw is that at low currents the transconductance of the output
devices is small. As the current increases ( which depends on the load as well
as output voltage ) the transconductance increases. This variation in
transconductance is what causes distortion as the 2 output devices cross over.

The larger the load current ( lower load resistance ) the greater the variation
and hence the greater the distortion.

Graham