Q.Explain the working of direct coupled amplifier.

In a direct coupled amplifier the collector of one stage is joined straight to the base of the next, with no capacitor or transformer in between, so it can pass DC and very-low-frequency signals. …

[!TLDR]

A direct coupled amplifier connects the collector of one stage straight to the base of the next; with no coupling capacitor it amplifies DC and very-low-frequency signals, the overall gain being the product of the stage gains.

Why direct coupling: some applications need extremely low frequency (below about 50 Hz) or DC signals to be amplified. Coupling capacitors or transformers cannot be used here because their size (and, for a capacitor, its high reactance) becomes very large at such low frequencies. Direct coupling avoids this by joining the stages directly.

Circuit: a two-stage amplifier using identical npn transistors. The collector of the first transistor is connected directly (no capacitor) to the base of the second transistor; the collector of the last stage feeds the load directly. $R_1$ and $R_2$ set the bias of the first transistor and also help bias the second, while $R_C$ and $R_E$ provide the necessary bias. …