Understanding DC Amplifier Characteristics

A DC amplifier, also known as a direct-coupled amplifier, is specifically designed to amplify signals that do not change over time, including zero-frequency (DC) signals. Let's examine the properties mentioned in the options to understand why the correct answer is 'All of these'.

Capability of Amplifying DC Signals

The primary function of a DC amplifier is to amplify direct current (DC) signals or signals with very low frequencies. Unlike AC amplifiers that use coupling capacitors to block DC components, DC amplifiers are built to pass these steady voltage or current levels without distortion.

Input Design Without Coupling Capacitors

To ensure that DC signals can be passed from one stage to the next without attenuation, DC amplifiers typically omit coupling capacitors (like CC) or coupling transformers at their input and between stages. This direct coupling allows the DC level of the input signal to be amplified and transferred directly to the output.

Thermal Stability Considerations

Direct-coupled amplifiers can sometimes exhibit poor thermal stability. This is because the operating point (Q-point) of the active devices (like transistors) used in the amplifier can shift due to temperature variations. Changes in parameters such as $I_{CEO}$ (collector-to-emitter cutoff current) with temperature can lead to unwanted drift in the output voltage or current, making the amplifier less stable thermally. Techniques like negative feedback or specific biasing circuits are often employed to improve thermal stability.

Conclusion on DC Amplifier Properties

Based on the analysis:

• DC amplifiers are indeed capable of amplifying DC signals.
• They are designed without the use of coupling capacitors to allow DC signal transfer.
• They can potentially suffer from poor thermal stability due to temperature effects on device parameters.

Since all the individual statements (1, 2, and 3) are true characteristics associated with DC amplifiers, the option stating 'All of these' is the correct choice.