# Why is LTSpice Bode plot disagreeing with a linear plot? Here is a picture of an amplifier that I'm simulating using LTSpice. When I insert a sine wave of 0.01 volts and 314Hz as an input, I get this: The output is taken from the collector and is oscillating between about 2.65 and 2.35 volts. This would mean that the amplitude of the output is about 150mV. This divided by the input amplitude of 10mV gives a gain of 15.

When I do a Bode plot of its frequency response, using 0.01V again as input, I get this: Past 100Hz, I get a gain of about -16.4 decibels! This is nothing like the gain of 15 that is claimed by the linear graph. When I convert -16.4 decibels to a gain ratio, I get something like 0.15. So what am I doing wrong?

• If you're not comfortable with AC analysis's "dbV" scale, you can click on that vertical scale (at the left) and change it to "logarithmic volts" or to "linear volts". – glen_geek Jul 20 '19 at 14:27
• Plot "V(out)/V(in)" otherwise you are plotting the amplitude in dBV – sstobbe Jul 20 '19 at 15:45

• @S.Rotos: Spice assumes that the input source that is used in AC analysis has an amplitude of 1V. $$A_v = \frac{V_{out}}{1 V}$$ So, when you start reducing this, it looks like it has a lower gain on the plot. – Linkyyy Jul 20 '19 at 16:11
$$10^{-16.4/20} = 151 \;\mbox{mV}$$