# Amplify AC signal only

I amplify a signal which including AC and DC.Why is the output waveform changed when C1 is 0.1u?And how to decide the value of C1?

When Capacitor Reactive Impedance, Xc(f) rises to affect impedance ratio with R such that they are equal, you can measure many responses;

1. the voltage amplitude divider ratio (gain) reduces times $$\\dfrac{1}{\sqrt(1+1)} = 0.707 = -3~dB\$$
2. the current phase shift changes by 45 deg, (=trig. angle with equal sides of impedance) which as already started to shift with 0.1uF that raises the impedance of Xc(f) to 26.5 kohm
3. the -3dB breakpoint is $$\ω_1=1/(R_1C_1)=2\pi f\$$
4. you can visualize all of this with a log impedance RLC nomograph and measure the impedance ratio of any XL(f) or C(f) vs V as the division ratio becomes a difference on a log scale. like 1000 is a difference of 3 decades
5. R+ C Series Impedance = $$\Zc(f)=R+jXc(f)\$$

1. Also with this you can "ballpark" estimate corner frequencies and LC resonant circuits and Q gain factors for LC intersections with R differences (Ratios). e.g. L//C//R is high impedance
so $$Q_p=R/X_L(f)=R/X_C(f)$$
2. at LC value intersection (=resonant f)
then series resonant f (L+C+R) $$Q_s=\frac{X_L(f)}{R}=\frac{X_C(f)}{R}$$ (using absolute values or ignoring phase due to j

Here is one example of the RLC Impedance nomograph.

• sorry for the late reply.if i want to avoid attenuation and phase shift,the impendance of capacitor must be very lower than 47k.is that right?
– 甚麼甚
Jan 24, 2020 at 4:17
• Usually 1 or 2 decade below @ f falstad.com/afilter/… Jan 24, 2020 at 4:33
• tinyurl.com/umlnwmo Jan 24, 2020 at 4:48