Q.Explain with the circuit diagram, the working of Direct coupled CE amplifier. Also explain the frequency response.

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

[!TLDR]

A direct-coupled CE 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, and its frequency response is flat from DC, rolling off only at high frequency.

Why direct coupling: to amplify extremely low frequency (below about 50 Hz) or DC signals, coupling capacitors and transformers are unsuitable because their size becomes very large and a capacitor's reactance becomes very high at such frequencies. Direct coupling joins the stages without any coupling element.

Circuit: a two-stage amplifier using npn transistors in CE configuration. The collector of the first transistor is connected directly to the base of the second transistor (no coupling capacitor); the collector of the last stage feeds the load directly. $R_1$ and $R_2$ provide the voltage-divider bias for the first transistor and also help bias the second, while $R_C$ and $R_E$ supply the necessary bias. No bypass or coupling capacitors are used.

Working: a signal applied to the base of the first transistor is amplified $A_1$ times and appears at its collector. Since this collector is directly joined to the base of the second transistor, the amplified signal becomes the second stage's input and is amplified a further $A_2$ times. The overall gain is $A=A_1\times A_2$. …