Difference Between AC & DC Amplifiers

If you’ve been trying recently to get a deeper understanding of power amplifiers, you have probably come across terms AC amplifier and DC amplifier. What was your first thought? Was it Alternating Current and Direct Current? Well, terms AC and DC in amplifiers don’t exactly refer to current. These terms refer to the coupling method between different stages of amplification.

Our article is designed to help you understand the working principles of each of these two amplifier types, and explain the differences between AC and DC amplifiers.


What is The Meaning of AC and DC in AC/DC Amplifiers?

AC in AC amplifiers tells us that different stages of amplification are coupled with a capacitor or a transformer. These capacitors/transformers are, therefore, known as coupling capacitors/transformers. The purpose of coupling capacitors and transformers is to strip away any DC voltage and provide a clear path for the AC voltage.

AC-coupled power amps, especially solid-state amps are not as common as they used to be 40 years ago. Most tube amps, on the other hand, are AC-coupled. Some companies still use AC-coupling for solid-state amps (like McIntosh). 

McIntosh MA8900

McIntosh MA8900 – AC-Coupled Integrated amplifier

DC amplifiers are also known as Direct-Coupled amplifiers. The circuit of a direct-coupled amp looks much cleaner because capacitors and transformers are not used for coupling between two stages of amplification. 

DC coupling is very common these days. Most of today’s solid-state power amps and integrated amps are DC-coupled.

PS Audio Stellar S300

PS Audio Stellar S300 – DC-Coupled Power Amplifier

Before we discuss the differences and advantages of each amplifier type, let’s see how AC and DC-coupled amp circuits look like and how they work.

AC-Coupled (Capacitor-Coupled or Transformer-Coupled) Amplifiers

Capacitor-coupled (RC-coupled) amp circuit 

When two stages of amplification are connected to each other through a combination of capacitors and resistors, the amp circuit is considered capacitor-coupled or capacitively-coupled, or RC-coupled.

In the picture below, you can see the diagram of a two-stage RC-coupled transistor amplifier (transistor amplifier means that transistors Q1 and Q2 are used to amplify the signal). 

Two-stage capacitor-coupled amplifier circuit diagram

Two-stage capacitor-coupled (RC-coupled) amplifier circuit diagram

As you can see, the output of the first stage (Q1) is coupled to the input of the second stage (Q2) with a coupling capacitor C2. The output stage of the second transistor (Q2) is coupled to the load resistor (RL – speaker) with a coupling capacitor C3.

Resistors R1, R2, RE are there to provide voltage divider biasing for Q1 and Q2 and to stabilize the circuit. 

RC and RE are resistors that are supposed to reduce the VCC voltage by 50% (RC – collector resistor, RE – emitter resistor).

The capacitor C2 couples the output stage of Q1 to the input stage of Q2. Because of the C2 capacitor and RC resistor, this amp is considered RC-Coupled.

The capacitor C1 couples the input to the base terminal of the Q1 transistor (first stage input). The capacitor C3 couples the output of the transistor Q2 (second stage output) to the load resistor RL (your speakers).

A weak signal (that is supposed to be amplified) is applied to the base of Q1, which amplifies the signal and switches the phase of the signal (180° phase shift). The signal is then applied, through coupling capacitor C2, to the base of Q2 (second stage input). C2 eliminates the DC current and passes through the AC signal. Q2 performs further amplification and again switches the phase of the signal (another 180° phase shift), which means that the input signal and output signal are in phase (360° phase shift). The amplified signal passes through the capacitor C2, which also eliminates the DC component and the amplified signal goes to the load resistor RL (speakers).

Transformer-coupled amp circuit

When two stages of the amplifier are coupled with a transformer, we call that circuit transformer-coupled. One of the most important characteristics of transformers is that they are great for impedance matching. So, if two stages have different impedances, adding a transformer with the right number of primary and secondary windings will allow you to match the impedances of the two stages (output of the first stage to the input of the second stage).

In the image below, you can see the diagram of a two-stage transformer-coupled amplifier circuit. 

Two-stage transformer-coupled amplifier circuit diagram

Two-stage transformer-coupled amplifier circuit diagram

As you can see, the output of the first stage (Q1) is coupled to the input of the second stage (Q2) with a coupling transformer T1. The output stage of the second stage (Q2) is coupled to the load resistor RL (speaker) with a coupling transformer T2.

Resistors R1, R2, RE are there to provide voltage divider biasing for Q1 and Q2 and to stabilize the circuit. 

The capacitor C1, located on the input, is there to eliminate DC voltage and enable the AC signal to pass through.

Capacitor CE is there to additionally stabilize the circuit and enable a low-reactance path to the signal. It is connected across the resistor RE (in both stages) and it acts as an emitter bypass capacitor – it bypasses emitter current to the ground so there’s less voltage drop across the RE. Because of that, the voltage gain increases. 

When an input signal (VIN) is applied to the base of transistor Q1 through capacitor C1, the signal is amplified and goes to the primary winding of T1. With the proper number of winding turns, you can maximize the energy flowing from the primary to the secondary winding of T1. 

The signal then goes from the secondary winding of T1 to the base of the transistor Q2 (the input of the second stage). Q2 provides further amplification and sends it to the primary winding of T2. The energy flows from the primary to the secondary winding and then is applied to RL (load resistance – speaker). Again, if the number of winding turns is carefully calculated, the maximum energy will be transferred to the RL.

DC (Direct-Coupled) Amplifiers

When there are no transformers or capacitors between the two stages of amplification, the amplifier is direct-coupled or DC-coupled. The circuit diagram of a direct-coupled amp is the simplest and cleanest. DC-coupled amps are usually used to amplify low-frequency signals. You can see it in the image below. 

Two-stage direct-coupled amplifier circuit diagram

Two-stage direct-coupled amplifier circuit diagram

So, as you can see, the output of the first stage (transistor Q1) is directly connected to the input of the second stage (transistor Q2). There are no coupling transformers or capacitors.

Voltage divider biasing is applied to the first stage (R1, R2, RE). This is necessary to stabilize the circuit and keep the Q point in the center of the load line. 

The output of the first stage is directly given to the second stage input. VCC voltage (DC voltage) is applied, and the output is taken from the stage two across the load resistor RL (speaker).

When a weak input signal is applied to the base of the Q1 (this is a low-frequency signal), Q1 amplifies the signal and its output is available across the collector resistor (RC). The output of the first stage is directly given to the stage two input, which performs further amplification and sends the output signal to the load resistor RL

Advantages and Disadvantages of AC Amplifiers

RC-Coupled

RC-coupled amplifiers are not too expensive or too complicated to build, which makes them quite popular. RC-coupled amps provide constant amplification (steady gain) across the audible frequency spectrum (20 Hz – 20 kHz) and the frequency response across that spectrum is very good. 

When it comes to downsides, it’s important to mention poor impedance matching (low-impedance input and high-impedance output). Also, their power gain is relatively low and the power transfer is low because of poor impedance matching. 

Because of their frequency response and great audio fidelity, they are often used in the audio industry. RC-coupling is used in voltage amplifiers, too. 

Transformer-Coupled

Transformer-coupled amps have a higher gain than the RC-coupled ones (10-20x higher).  Their most important advantage is impedance matching (achieved through using transformers with the right number of turns on the primary and secondary windings). That way, you can match the low output impedance of one stage to the high input impedance of the other stage.

Transformer-coupled amps are also very efficient – they don’t create significant power loss.

When it comes to downsides, you need to know that transformer-coupled amps don’t have a steady gain like RC-coupled amps. Their gain varies significantly with the frequencies and it’s particularly low for very low and very high frequencies. To conclude, their frequency response is far from great. 

Transformers also introduce a noticeable humming noise. They are quite bulky and heavy. And they are also quite expensive. 

Due to their upsides, they are used in systems where you have to match the impedance of different stages and deliver max power to the output device (speakers, for example).

Advantages of Direct-Coupled Amplifiers

DC-coupled amps are the cheapest to manufacture since there’s no need for expensive transformers and capacitors. Because of the lack of additional components, they are also the simplest, the most compact, and the lightest. Their impedance matching is decent (not as good as transformer-coupled amps but better than RC-coupled amps). Their frequency response is very good, especially when it comes to very low frequencies. 

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The most important downsides are poor high-frequency amplification and high sensitivity to temperature changes. Their output also varies over time.

SUMMARY

To sum things up, here’s a simple table explaining the advantages and disadvantages of RC-coupled, transformer-coupled, and direct-coupled amplifiers.

CATEGORYRC-CoupledTransformer-CoupledDirect-Coupled
CostIn the middleMost ExpensiveCheapest
Size and WeightIn the middleLargest and heaviestSmallest
Impedance MatchingNot so goodGreatPretty good
Frequency ResponseExcellent for the audible rangeNot so greatBest
UseVoltage amplificationPower amplificationBest for amplifying low frequencies

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