A device for amplifying signals with direct-current components. There are many different situations where it is necessary to amplify signals having a frequency spectrum which extends to zero. Some typical examples are amplifiers in electronic differential analyzers (analog computers), certain types of feedback control systems, medical instruments such as the electrocardiograph, and instrumentation amplifiers. Amplifiers which have capacitor coupling between stages are not usable in these cases, because the gain at zero frequency is zero. Therefore, a special form of amplifier, called a dc (direct-current) or direct-coupled amplifier, is necessary. These amplifiers will also amplify alternating-current (ac) signals.