I Need To Compute The Voltage Rating For Capacitor C1

Thread Starter


Joined Mar 31, 2017
I am designing a complementary-symmetry power amplifier. The Supply voltages are +/-
12 volts. The point at which the signal enters the amplifier has a voltage of zero dc. The
maximum input voltage is a 4 volt peak sine wave. How do I determine the required voltage
rating for the capacitor?



Joined Jul 20, 2014
From a technician/repairman's point of view: assume that one of the transistors might fail and thus put an entire power supply rail at the capacitor. Then go one size bigger than that. So I'd go at least 16V. That's generic advice without any knowledge of what the stage before this is doing. Since I would assume both the input and output side of the capacitor are normally going to be at 0V (ground) the cap will probably need to be a nonpolar type.

All of THAT assumes that you're married to this output stage. I would much rather use a directly-coupled version driven from the collector of a gain stage and skip the cap altogether (also lets you use more negative feedback = less distortion overall. This version of the complementary output will cost you some gain.)


Joined Aug 1, 2013
The basic rule of thumb for long term reliability is that capacitors should be overrated by 100%. So if the worst case peak voltage is 12 V, use a 25 V cap.

Separate from that, there is an AC voltage developed across the cap during normal operation. With the cap value and the input signal bandwidth, you can calculate the capacitor's worst case impedance. With that you can calculate the current into D1/D2, and with that you can calculate the AC voltage across the cap.