TRANSISTOR AUDIO FREQUENCY AMPLIFIERS. POWER AMPLIFIERS
By the request of site visitors, I present an article entirely dedicated to transistor amplifiers. In, we briefly touched upon the topic of transistor amplification stages, so with this article, I will try to fill in all the gaps regarding transistor amplifiers. Some of the theoretical foundations presented here are valid for both transistor and vacuum tube amplifiers. At the beginning of the article, we will overview the main types and methods of connecting amplifier stages. Later, we will look at the main pros and cons of single-ended transformer and transformerless amplifiers. We will also pay special attention to push-pull transformer and transformerless amplifiers, as they are widely used and of great interest. Finally, just like in previous lessons, there will be practical work at the end. This article does not differ from the regular lessons, with the only difference being that this and all subsequent articles will have specific titles, allowing you to choose a topic to study. In any case, to confidently tackle any of the upcoming topics, you must complete the full basic course consisting of 10 lessons.
A transistor amplifier stage is a transistor combined with resistors, capacitors, and other components that provide the necessary conditions for it to work as an amplifier. For loud reproduction of audio frequency oscillations, a transistor amplifier must be at least two or three stages long. In multi-stage amplifiers, we distinguish between preamplifier stages and output (or final) stages. The output stage is the last stage of the amplifier that drives the headphones or loudspeaker, and all the stages before it are preamplifiers. The task of one or more preamplifier stages is to increase the audio frequency voltage to the level required to drive the output stage transistor. The output stage transistor is required to increase the power of the audio frequency oscillations to the level needed to drive the dynamic speaker. For the output stages of the simplest transistor amplifiers, radio amateurs often use the same low-power transistors as in the preamplifier stages. This is done to make the amplifiers more power-efficient, which is especially important for portable, battery-powered devices. The output power of such amplifiers is small — from a few tens to 100 - 150 mW, but even this is enough to drive headphones or low-power speakers. However, if energy conservation is not a major concern, for example, when powering amplifiers from the AC mains, powerful transistors are used in the output stages. What is the operating principle of a multi-stage amplifier? You can see the schematic of a simple two-stage AF transistor amplifier in (Fig. 1). Look at it carefully. Transistor V1 works in the first stage, and transistor V2 in the second. Here, the first stage is the preamplifier, and the second is the output stage. Between them is a coupling capacitor C2. The operating principle of either stage is identical and similar to the single-stage amplifier you are already familiar with. The only difference is in the components: the load of the first stage transistor V1 is resistor R2, while the load of the output stage transistor V2 is the headphones B1 (or a speaker if the output signal is powerful enough). The bias for the base of the first transistor is supplied through resistor R1, and for the second through resistor R3. Both stages are powered by a common source (Ucc), which can be a battery or a power supply. The operating points (biasing) of the transistors are set by selecting the values of resistors R1 and R3, which is indicated on the schematic by asterisks.
Fig. 1 Two-stage transistor amplifier.
The operation of the amplifier as a whole is as follows. The electrical signal, fed through capacitor C1 to the input of the first stage and amplified by transistor V1, goes from the load resistor R2 through the coupling capacitor C2 to the input of the second stage. Here it is amplified by transistor V2 and converted into sound by the headphones B1 connected in the transistor's collector circuit. What is the role of capacitor C1 at the amplifier's input? It performs two tasks: it freely passes the AC signal voltage to the transistor and prevents the base from shorting to the emitter through the signal source. Imagine that this capacitor is missing, and the signal source is a dynamic microphone with low internal resistance. What would happen? The base of the transistor would be connected to the emitter through the low resistance of the microphone. The transistor would close (turn off) because it would operate without an initial DC bias voltage. It would only open during the negative half-cycles of the signal voltage. And the positive half-cycles, which close the transistor even more, would be clipped. As a result, the transistor would heavily distort the amplified signal. Capacitor C2 couples the amplifier stages for alternating current. It must easily pass the AC component of the amplified signal and block the DC component of the first stage's collector circuit. If the capacitor conducts direct current along with the AC component, the operating mode of the output stage transistor will be disrupted, and the sound will become distorted or disappear completely. Capacitors that perform these functions are called coupling or DC blocking capacitors. Input and coupling capacitors must efficiently pass the entire frequency band of the amplified signal — from the lowest to the highest. Capacitors with a capacitance of at least 5 µF meet this requirement. The use of large-capacity coupling capacitors in transistor amplifiers is due to the relatively low input impedance of transistors. The coupling capacitor offers a capacitive reactance (AC resistance) to the alternating current, which is smaller the larger its capacitance. If its reactance is greater than the input impedance of the transistor, a larger portion of the AC signal voltage will drop across it rather than across the transistor's input, leading to a loss in amplification. The capacitive reactance of the coupling capacitor should be at least 3 to 5 times less than the input impedance of the transistor. This is why large-capacity capacitors are used at the input and for interstage coupling. Usually, compact electrolytic capacitors are used here, and their polarity must strictly be observed. These are the most characteristic features of the components in a two-stage AF transistor amplifier. To solidify your understanding of how it works, I suggest building, tuning, and testing the simplest amplifier circuit variants shown below. (At the end of the article, practical work options will be offered; for now, you should assemble a prototype of a simple two-stage amplifier so you can observe the theoretical statements in practice).
Simple, Two-Stage Amplifiers
The schematic diagrams for two variants of such an amplifier are shown in (Fig. 2). They are essentially a repetition of the transistor amplifier circuit we just discussed. However, they include component values and three additional elements: R1, C3, and S1. Resistor R1 is the load for the audio frequency source (e.g., a crystal radio or phonograph pickup); C3 is a capacitor that bypasses higher audio frequencies for the speaker B1; S1 is the power switch. The amplifier in (Fig. 2, a) uses PNP transistors, while the one in (Fig. 2, b) uses NPN transistors. Because of this, the polarity of their power supply batteries is different: a negative voltage is applied to the collectors of the first variant, and a positive voltage to the collectors of the second variant. The polarity of the electrolytic capacitors is also different. Otherwise, the amplifiers are completely identical.
Fig. 2 Two-stage AF amplifiers using PNP transistors (a) and NPN transistors (b).
Transistors with a DC current gain (hFE or h21E) of 20 - 30 or more can work in any of these variants. A transistor with a higher hFE should be placed in the preamplifier stage (the first one). Headphones or a telephone earpiece (like the DEM-4m) can serve as the load B1 for the output stage. To power the amplifier, use a 4.5V flat battery (like the 3R12). First, assemble the amplifier on a breadboard, after which you can transfer its components to a printed circuit board if you wish. First, mount only the components of the first stage and capacitor C2 on the breadboard. Connect the headphones between the right side (according to the schematic) of this capacitor and the grounded wire of the power supply. If you now connect the amplifier input to the output jacks of, for example, a crystal radio tuned to a station, or connect any other weak signal source to it, you will hear the broadcast or signal in the headphones. By adjusting the resistance of resistor R2, achieve the maximum volume. In this case, a milliammeter connected to the transistor's collector circuit should show a current of 0.4 - 0.6 mA. With a 4.5V power supply, this is the optimal operating mode for this transistor. Next, mount the components for the second (output) stage, connecting the headphones to its transistor's collector circuit. Now the headphones should sound much louder. They might sound even louder if you adjust resistor R4 to set the transistor's collector current to 0.4 - 0.6 mA. You can take a different, more technical approach: assemble all the amplifier components, set the recommended transistor operating modes (using collector currents or voltages) by adjusting resistors R2 and R4, and only then test it for sound reproduction. For more complex amplifiers, which you will mostly deal with, this is the only correct way. I hope you understand that my advice on tuning applies equally to both variants. And if the current gain of their transistors is approximately the same, the volume of the headphones should also be identical. With a DEM-4m earpiece (which has an impedance of 60 Ohms), the quiescent current of the stage's transistor must be increased (by reducing R4's resistance) to 4 - 6 mA. The schematic of a third variant is shown in (Fig. 3). A special feature of this amplifier is that it uses a PNP transistor in the first stage and an NPN transistor in the second. Moreover, the base of the second transistor is connected to the collector of the first not through a coupling capacitor, but directly, or as they say, galvanically (DC coupling). With this type of coupling, the frequency range of the amplified oscillations is expanded, and the operating point of the second transistor is largely determined by the first one, which is set by adjusting resistor R2. In such an amplifier, the load for the first transistor is not resistor R3, but the emitter-base junction (PN junction) of the second transistor. The resistor is only needed for biasing: the voltage drop across it opens the second transistor. If this is a germanium transistor, R3 can be 680 - 750 Ohms, and if it's silicon, around 3 kOhms. Unfortunately, the stability of this amplifier to changes in power supply voltage or temperature is low.
Fig. 3 Amplifier using transistors of different polarities (PNP and NPN).
Mounting the components of a tuned amplifier onto a permanent board is a simple task. For example, (Fig. 4) shows the layout board for the first amplifier variant (based on the schematic in Fig. 2, a). Cut the board from a 1.5 - 2 mm thick sheet of fiberglass or pertinax. The dimensions shown are approximate and depend on the size of your components.
Fig. 4 Layout board of the two-stage AF amplifier.
Stabilizing the Transistor's Operating Point (Biasing)
An amplifier assembled and tuned indoors will work better than outside, where it might be exposed to the hot summer sun or winter frost. Why? Because, unfortunately, as temperature rises, the transistor's operating point shifts. The root cause is the uncontrolled collector cutoff current (Icbo) and changes in the DC current gain (hFE) with temperature. In principle, the Icbo current is small. But it increases significantly with temperature. There is a way to stabilize the transistor's operating point with somewhat fewer losses in amplification, but it requires making the stage slightly more complex. The schematic for such an amplifier is shown in (Fig. 5, b). The quiescent mode remains the same: the collector current is 0.8 - 1 mA. But it is set using two additional resistors: Rb2 and Re. Resistors Rb1 and Rb2 form a voltage divider that maintains a stable voltage at the base. The emitter resistor Re acts as the thermal stabilization element. Thermal stabilization works as follows: as the collector current increases due to heat, the voltage drop across resistor Re increases. This reduces the voltage difference between the base and emitter, which automatically lowers the collector current. This creates negative feedback between the emitter and base, stabilizing the transistor's operating point. Capacitor Ce provides a bypass path for the AC component of the collector current, preventing AC negative feedback and preserving the stage's gain.
Fig. 5 Amplifier stages with transistor thermal stabilization.
Push-Pull Power Amplifier
Earlier, I mentioned that radio amateurs use the same low-power transistors in the output stages as in the voltage amplification stages. How is this achieved? These stages are called push-pull power amplifiers. They can be either transformer-coupled or transformerless. A simplified diagram of a push-pull transformer power amplifier and graphs illustrating its operation are shown in (Fig. 6). As you can see, there are two transformers and two transistors. Transformer T1 is interstage, and T2 is the output transformer. Transistors V1 and V2 are connected in a Common Emitter (CE) configuration. Each transistor and its corresponding sections of the transformers act as a normal single-ended amplifier. Together, they form a push-pull power amplifier.
Fig. 6 Transformer push-pull power amplifier and graphs illustrating its operation.
The essence of a push-pull amplifier's operation is as follows. Audio frequency oscillations (graph a in Fig. 6) from the preamplifier are fed to the bases of both transistors so that their voltages change in opposite directions, i.e., completely out of phase (anti-phase). The transistors work alternately, in two "pushes/pulls" for each period. When, for instance, a negative half-wave is at the base of transistor V1, it opens, and current flows through its section of T2 (graph b). At this time, V2 is closed. In the next half-cycle, V2 opens and V1 closes (graph c). In the transformer winding, the collector currents are summed (graph d), resulting in a much more powerful electrical audio signal at the output. Now, let's look at the transformerless push-pull power amplifier (Fig. 7). There are also two transistors, but of different polarities (complementary pair): V1 is PNP, and V2 is NPN. Because they have different polarities, they naturally operate alternately on the two half-cycles. You get the same effect as with transformers, but the complementary pair eliminates the need for an external phase splitter.
Fig. 7 Transformerless push-pull power amplifier.
You may have noticed one contradiction: no bias voltages were applied to the bases of the transistors in my explanation. You are right. Push-pull transistors *can* operate without initial bias. But then, a specific type of distortion called crossover distortion (ступенька) appears, which is very noticeable at low volume. It gets its name because, on an oscilloscope, the sine wave looks like it has steps near the zero-crossing (Fig. 8). To eliminate this, a small forward bias voltage is applied to the bases in practical circuits.
Fig. 8 Crossover distortion (Step distortion).
MAIN PARAMETERS OF AF AMPLIFIERS
The quality and suitability of an amplifier are judged by several parameters, the three most important being: Output power (Pout), Sensitivity, and Frequency Response.
Output power is the electrical power of the audio signal, expressed in Watts (W) or milliwatts (mW), delivered to the load. We distinguish between nominal and maximum power. Nominal power is the level where non-linear distortion does not exceed 3 - 5%. Maximum power is where distortion hits 10% (often very noticeable to the ear).
Sensitivity is the audio signal voltage (in Volts or millivolts) that must be applied to the input to achieve nominal output power. The smaller this voltage, the better the sensitivity.
Frequency Response (Amplitude-Frequency Characteristic - AFC) is graphically represented by a line showing the dependence of the output voltage on frequency for a constant input voltage. Any real amplifier will have a roll-off at the extreme low and high ends of the audio spectrum.
Amplifier Classes and Efficiency
In a single-ended amplifier operating in Class A, the maximum theoretical efficiency is 50%, but in reality, it rarely exceeds 30 - 40%. This means the battery has to supply more than twice the power that actually reaches the speaker (Fig. 9).
Fig. 9 The higher the efficiency, the less power is consumed from the power supply.
Amplification without cutoff, where the AC component never exceeds the quiescent current, is called Class A. If the current is cut off for exactly half the cycle, we have Class B. If it conducts for slightly more than half the cycle, it's Class AB.
In a push-pull amplifier, we split the signal into two halves and amplify them separately in Class B or AB (Fig. 10). The theoretical efficiency of Class B is 78.5% (practically around 67%). Thus, every watt of audio in a push-pull amplifier costs us two to three times less battery power than in a single-ended one.
Fig. 10 Push-pull stage and amplification classes.
To drive a push-pull circuit using identical transistors, we need a phase inverter (phase splitter), like a transformer with a center-tapped secondary (Fig. 11), to provide the two anti-phase signals.
Fig. 11 A phase inverter creates two AC voltages shifted by 180 degrees.
Fig. 12 Approximate Frequency Response graph of an amplifier.
Another crucial concept is Negative Feedback (NFB). NFB feeds a portion of the output signal back to the input in anti-phase, which reduces non-linear distortion and can also be used to shape the frequency response (tone control). Fig. 13 and Fig. 14 show practical schematics of transformerless and transformer push-pull amplifiers implementing these concepts.
Fig. 13 Practical schematic of a transformeress push-pull amplifier.
Fig. 14 Push-pull AF amplifier with a transformer output stage.
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