There are many applications in which extremely low frequency (< 10 Hz) signals are to be amplified e.g. amplifying photo-electric current, thermo-couple current etc. The coupling devices such as capacitors and transformers cannot be use because the electrical sizes of these components become very large at extremely low frequencies. Under such situations, one stage is directly connect to the next stage without any intervening coupling device.
This type of coupling is know as direct coupling. Circuit details.
Fig. 1 shows the circuit of a three-stage direct-coupled amplifier. It uses *complementary transistors. Thus, the first stage uses npn transistor, the second stage uses pnp transistor and so on. This arrangement makes the design very simple. The output from the collector of first transistor T1 is feed to the input of the second transistor T2 and so on.
This makes the circuit stable w.r.t. temperature changes. In this connection (i.e., npn followed by pnp), the direction of collector current increase β, when the temperature rises, is opposite for the two transistors. Thus the variation in one transistor tends to cancel that in the other.
The weak signal is apply to the input of first transistor T1. Due to transistor action, an amplified output is obtain across the collector load RC of transistor T1. This voltage drives the base of the
second transistor and amplified output is obtained across its collector load. In this way, direct coupled
amplifier raises the strength of weak signal.
(i) The circuit arrangement is simple because of minimum use of resistors.
(ii) The circuit has low cost because of the absence of expensive coupling devices.
(i) It cannot be use for amplifying high frequencies.
(ii) The operating point is shift due to temperature variations.