An amplifier which is simply termed as the amp is the most essential component in the electronics industry. Not only the amplifier boosts the signal level, the device even delivers an enhanced quality of the input signal. To boost up voltage level, preferably transformers are used, while a relay is utilized for switching ON or OFF of electric pulses. And to magnify the range of fluctuating signals, transistor-based amplifiers are used. So, we are implementing the purpose of amplifiers in almost every purpose. And today, this article describes DC amplifier (Direct Coupled), their characteristics, working phenomenon, circuit, applications, and benefits.
What is Direct Coupled Amplifier?
A DC amplifier is the device where the output one stage is directly coupled to the input od the next stage so that even zero range of frequency signals are permitted. For the amplification purpose of a minimal range of frequencies, like thermocouple or photoelectric currents, these DC amplifiers are used. As the name itself specifies that, there is no need of any kind of coupling device to couple input and output stages.
The only means of DC amplifiers is not for direct current amplification, it can be even employed for both alternating and direct current amplification. As the frequency response of the direct-coupled amplifier is analogous to LFP, this is even termed as a low pass amplifier. DC amplifiers got more prominence as because these are the only suitable devices for the amplification of zero frequency signals.
DC Amplifier Characteristics
The characteristics of dc amplifier include the following.
- The frequency response range of DC amplifiers is the best
- The constructional equipment and cost is not expensive
- The design of DC amplifiers is so streamlined
- Not much weight and space are necessary
- When compared with another coupling method, the direct coupling method offers good impedance matching
Construction of DC Amplifier
Here is the constructional design of the two-stage DC amplifier circuit. The input of the second transistor which is T2 is connected to the output of the first transistor T1. For the initial stage, an NPN transistor is chosen and for the next stages, PNP transistors are used. Because of this approach, the changes that happened in one transistor will lead to drop off other transistor changes. The increase in collector current and the change in the value of ‘β’ in one transistor will be balanced by the reduction in the other transistor.
And the working of the DC amplifier is explained as that upon the application of input signal at the transistor T1 base end, it undergoes amplification because of transistor’s action and the output is delivered at the collector end f the T1. Then the output of T1 is applied as input to T2 and it undergoes further amplification. So, in this manner, DC amplifiers amplify signals through the direct coupling. To be clear, when the positive half cycle is applied as input to T1, it forward bias the signal and delivers an inverted signal at the output where
VcE = VCC – ICRC
This inverted signal is applied as input to T2 and this is self-biased as because they are in cascaded connection. The base-end of T2 will not conduct and it again reverses the applied input signal. When T1 does not undergo conduction, then the voltage drop across CE junction will be null, then VCC quals to ICRC.
The direct-coupled amplifier circuit can be shown as below:
DC Amplifier Circuit
Direct Coupled Class Amplifier
Of all the entire amplifier topology, the most implemented kind of amplifiers is a direct-coupled class A amplifier. The constructional design of this kind is very simple as they use only one switching transistor either IGBT, bipolar, or FET. As the transistor is biased all about the scope of Q point inside to the load centerline, it will never move either into saturation or cut-off positions. This scenario allows the conduction of current all through negative and positive cycles of input. The disadvantage that is in this type of amplifier is that the output transistor will never goes into OFF mode.
DC class A amplifier
These are considered as the best because of their linearity conditions, excellent gain, and also the minimal level of distortion. Even these are less considered for high power amplification purposes, direct-coupled Class A amplifiers are the best of all. The best-implemented application of this kind is in increased fidelity of audio amplifiers. The circuit is shown below:
In order to maximize gain and linearity levels, the output phase should be in ON condition’. When the null signal idle current value in the output section is either greater than or similar to the load current, then the amplifier is considered as class A. As this kind of amplifiers functions in the linear segment of the performance curves, then the one output device allows conductions all through the waveform which makes this device similar as the current source. The frequency response curve is:
Frequency Response Of Class A Amplifier
As DC class A amplifiers functions in the linear region, the base-end of the transistor has to be selected correctly to make sure for proper functionality and minimal distortion. Even because of this added benefit, there is a major disadvantage that continuous ON condition of output transistor leads to power loss. As because of this power loss, the amplifiers generate an extensive amount of heat and thus reduces the efficiency by 30 percent. This impacts on high-power applications. Also because of maximum idling current levels, the power supply has to be chosen correctly and proper filtration has to be done to eliminate any kind of noise and distortions.
Advantages/Disadvantages of Direct Coupled Amplifier
There are extensive benefits of employing a direct-coupled amplifier and those are stated as
- As because a minimal number of components are there in the constructional design of DC amplifiers, the arrangement and size of the circuit are less.
- Because of less circuitry arrangement, the cost is also less
- It is employed to boost up the levels of either minimal or zero level frequency signals
- Also, there are a smaller number of resistors usage in the circuit.
Disadvantages
Apart from having more benefits, DC amplifiers also have disadvantages and those are
- There will be some DRIFT voltage in the DC amplifiers which leads to annoying variation in the output voltage level.
- With respect to time or voltage, the output value changes in correspondence to the input voltage
- Parameters such as β and Vbe will change as per temperature variations. This leads to variation in the voltage and collector current values.
- When there is any noise or distortions in the input, it will be reflected at the output and this noise will also get increased because of high gain.
Comparison of Coupling Methods
When various coupling methods like a transformer, direct and RC couplings are
| Feature | DC | Transformer | RC |
| Space Requirement and Weight | Less required | More space necessity | Minimal |
| Impedance Matching | Best when compared with others | Best of all others | Very less |
| Cost | Minimal | More expensive | Poor |
| Frequency Response | Good frequency response curves | Less | Good in the audio range |
| Utilization | For the amplification of minimal level of frequency signals | For the usage of power amplification | For the purpose of the voltage of boost up |
Applications of DC Amplifier
Few of the applications of DC amplifier are:
- Employed in TV receivers
- Computers
- Electronic devices like speakers, Audio amplifiers and others
- Regulation devices
- Differential, pulse, and operational amplifiers
Know more about DC Circuit MCQs, Operational Amplifier MCQs.
So, this is all about the detailed concept of direct-coupled amplifiers. As because of many advantages these stand as fundamental for many amplifiers and for many electronic instruments. To gain more knowledge related to this concept know Why DC amplifiers hold more prominence and what are the frequency response of DC amplifiers?