Direct coupled
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Also known as DC Amp, in this type of amplifier, the output of one stage of the amplifier is connected to the input of the next stage directly without using any reactive components like capacitors, inductors, transformers, etc. Therefore the frequency response of this amplifier is independent of frequency.
DC Amp does not mean direct current amplifier, as this can be used for both d.c. and a.c. signals. The amplification of dc (zero frequency) is possible only by this amplifier, hence it later becomes the building block for differential amplifier and operational amplifier. Unlike other multistage amplifiers, amplification of low frequency (below 10 Hz) signals is only possible using direct coupling. This is because use of coupling and bypass capacitors is not possible as they provide very high capacitive reactance.
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[edit] Advantages
- Simple circuit arrangement as there are minimum number of components
- For above reason, also quite inexpensive
- Can be used to amplify zero and low frequency signals
- Provides uniform frequency response up to a high frequency. Lower cut-off frequency is zero and upper cut-off frequency is determined by values of components used.
[edit] Drawbacks
- Unwanted change in output voltage without any change in the input is often observed. This is caused by temperature variations, aging of components, etc and is referred to as 'drift'. This drawback can be removed using differential amplifier.
- At high frequencies gain decreases
- Transistor parameters change with temperature, hence output is affected by temperature variations
- Even if input is ac, some dc component is present at output. Can be removed using differential amplifier
- Any noise or stray pickup appearing at the input is also present at the output in amplified form, due to high gain
[edit] Applications
Used in TV receivers, computers, regulator circuits and other electronic instruments. Forms building block for differential amplifier and operational amplifier.
