The transistor amplifier, despite its already long history, remains a favorite subject of study for both beginners and veteran radio amateurs. And this is understandable. It is an indispensable component of the most popular amateur radio devices: radio receivers and low (sound) frequency amplifiers. We will look at how the simplest low-frequency transistor amplifiers are built.
Amp frequency response
In any television or radio receiver, in every music center or sound amplifier, you can find transistor sound amplifiers (low frequency - LF). The difference between audio transistor amplifiers and other types lies in their frequency response.
The transistor audio amplifier has a uniform frequency response in the frequency band from 15 Hz to 20 kHz. This means that all input signals with a frequency within this range are converted (amplified) by the amplifier.about the same. The figure below shows the ideal frequency response curve for an audio amplifier in the coordinates "amplifier gain Ku - input signal frequency".
This curve is almost flat from 15Hz to 20kHz. This means that such an amplifier should be used specifically for input signals with frequencies between 15 Hz and 20 kHz. For input signals with frequencies above 20 kHz or below 15 Hz, its efficiency and performance deteriorate rapidly.
The type of the frequency response of the amplifier is determined by the electrical radio elements (ERE) of its circuit, and above all by the transistors themselves. An audio amplifier based on transistors is usually assembled on the so-called low- and mid-frequency transistors with a total bandwidth of input signals from tens and hundreds of Hz to 30 kHz.
Amplifier class
As you know, depending on the degree of continuity of current flow throughout its period through the transistor amplifying stage (amplifier), the following classes of its operation are distinguished: "A", "B", "AB", "C", "D ".
In class of operation, current "A" flows through the stage for 100% of the input signal period. The cascade in this class is illustrated in the following figure.
In the class "AB" amplifier stage, the current flows through it for more than 50%, but less than 100% of the period of the input signal (see figure below).
In the class of operation of the "B" stage, the current flows through it exactly 50% of the period of the input signal, as illustrated in the figure.
Finally, in the "C" stage operation class, the current flows through it for less than 50% of the input signal period.
LF-transistor amplifier: distortion in the main classes of work
In the working area, the class "A" transistor amplifier has a low level of non-linear distortion. But if the signal has impulse surges in voltage, leading to saturation of the transistors, then higher harmonics (up to the 11th) appear around each “standard” harmonic of the output signal. This causes the phenomenon of the so-called transistorized or metallic sound.
If low-frequency power amplifiers on transistors have an unstabilized power supply, then their output signals are modulated in amplitude near the mains frequency. This leads to harshness of the sound at the left edge of the frequency response. Various voltage stabilization methods make the design of the amplifier more complex.
Typical efficiency of single-ended Class A amplifier does not exceed 20% due to the always-on transistor and the continuous flow of the DC component. You can make a class A amplifier push-pull, the efficiency will increase slightly, but the half-waves of the signal will become more asymmetric. The transfer of the cascade from the work class "A" to the work class "AB" quadruples the nonlinear distortion, although the efficiency of its circuit increases.
Bamplifiers of classes "AB" and "B" distortion increase as the signal level decreases. You involuntarily want to turn up such an amplifier louder for the full sensation of the power and dynamics of the music, but often this does not help much.
Intermediate job classes
Work class "A" has a variation - class "A+". In this case, the low-voltage input transistors of the amplifier of this class operate in class "A", and the high-voltage output transistors of the amplifier, when their input signals exceed a certain level, go into classes "B" or "AB". The efficiency of such cascades is better than in the pure class "A", and the non-linear distortion is less (up to 0.003%). However, they also sound "metallic" due to the presence of higher harmonics in the output signal.
Amplifiers of another class - "AA" have even lower degree of non-linear distortion - about 0.0005%, but higher harmonics are also present.
Return to Class A transistor amplifier?
Today, many specialists in the field of high-quality sound reproduction advocate a return to tube amplifiers, since the level of non-linear distortion and higher harmonics introduced by them into the output signal is obviously lower than that of transistors. However, these advantages are largely offset by the need for a matching transformer between the high-impedance tube output stage and the low-impedance speakers. However, a simple transistorized amplifier can be made with a transformer output as shown below.
There is also a point of view that only a hybrid tube-transistor amplifier can provide the ultimate sound quality, all stages of which are single-ended, not covered by negative feedback and work in class "A". That is, such a power follower is an amplifier on a single transistor. Its scheme can have the maximum achievable efficiency (in class "A") no more than 50%. But neither the power nor the efficiency of the amplifier are indicators of the quality of sound reproduction. At the same time, the quality and linearity of the characteristics of all EREs in the circuit are of particular importance.
As single-ended circuits get this perspective, we'll look at their options below.
Single-ended single-transistor amplifier
Its circuit, made with a common emitter and R-C connections for input and output signals for operation in class "A", is shown in the figure below.
It shows an n-p-n transistor Q1. Its collector is connected to the +Vcc positive terminal via a current-limiting resistor R3, and its emitter is connected to -Vcc. The p-n-p transistor amplifier will have the same circuit, but the power supply leads will be reversed.
C1 is a decoupling capacitor that separates the AC input source from the DC voltage source Vcc. At the same time, C1 does not prevent the passage of an alternating input current through the base-emitter junction of transistor Q1. Resistors R1 and R2 together with resistancetransition "E - B" form a voltage divider Vcc to select the operating point of the transistor Q1 in static mode. Typical for this circuit is the value of R2=1 kOhm, and the position of the operating point is Vcc / 2. R3 is a collector circuit load resistor and is used to create a variable voltage output signal on the collector.
Assume that Vcc=20 V, R2=1 kOhm, and the current gain h=150. We select the voltage at the emitter Ve=9 V, and the voltage drop at the transition "A - B" is taken equal to Vbe=0.7 V. This value corresponds to the so-called silicon transistor. If we were considering an amplifier based on germanium transistors, then the voltage drop across the open junction "E - B" would be Vbe=0.3 V.
Emitter current, approximately equal to collector current
Ie=9 V/1 kΩ=9 mA ≈ Ic.
Base current Ib=Ic/h=9mA/150=60uA.
Voltage drop across resistor R1
V(R1)=Vcc - Vb=Vcc - (Vbe + Ve)=20V - 9.7V=10.3V
R1=V(R1)/Ib=10, 3 V/60 uA=172 kOhm.
C2 is needed to create a circuit for the passage of the variable component of the emitter current (actually the collector current). If it were not there, then the resistor R2 would severely limit the variable component, so that the bipolar transistor amplifier in question would have a low current gain.
In our calculations, we assumed that Ic=Ib h, where Ib is the base current flowing into it from the emitter and arising when a bias voltage is applied to the base. However, through the base always (both with and without offset)there is also a leakage current from the collector Icb0. Therefore, the real collector current is Ic=Ib h + Icb0 h, i.e. the leakage current in the circuit with OE is amplified by 150 times. If we were considering an amplifier based on germanium transistors, then this circumstance would have to be taken into account in the calculations. The fact is that germanium transistors have a significant Icb0 of the order of several μA. In silicon, it is three orders of magnitude smaller (about a few nA), so it is usually neglected in calculations.
Single-ended MIS transistor amplifier
Like any field-effect transistor amplifier, the circuit in question has its analogue among bipolar transistor amplifiers. Therefore, consider an analogue of the previous circuit with a common emitter. It is made with a common source and R-C connections for input and output signals for operation in class "A" and is shown in the figure below.
Here C1 is the same decoupling capacitor, by means of which the AC input source is separated from the DC voltage source Vdd. As you know, any field-effect transistor amplifier must have the gate potential of its MOS transistors below the potentials of their sources. In this circuit, the gate is grounded by R1, which is typically high resistance (100 kΩ to 1 MΩ) so that it does not shunt the input signal. There is practically no current through R1, so the gate potential in the absence of an input signal is equal to the ground potential. The source potential is higher than the ground potential due to the voltage drop across the resistor R2. SoThus, the gate potential is lower than the source potential, which is necessary for the normal operation of Q1. Capacitor C2 and resistor R3 have the same purpose as in the previous circuit. Since this is a common-source circuit, the input and output signals are out of phase by 180°.
Transformer Output Amplifier
The third single-stage simple transistor amplifier, shown in the figure below, is also made according to the common emitter circuit for operation in class "A", but it is connected to a low-impedance speaker through a matching transformer.
The primary winding of transformer T1 is the collector circuit load of transistor Q1 and develops an output signal. T1 sends the output signal to the speaker and ensures that the output impedance of the transistor matches the low (on the order of a few ohms) speaker impedance.
The voltage divider of the collector power supply Vcc, assembled on resistors R1 and R3, provides the choice of the operating point of the transistor Q1 (supplying a bias voltage to its base). The purpose of the remaining elements of the amplifier is the same as in the previous circuits.
Push-pull audio amplifier
The two-transistor push-pull low-frequency amplifier splits the input audio signal into two out-of-phase half-waves, each of which is amplified by its own transistor stage. After such amplification, the half-waves are combined into a complete harmonic signal, which is transmitted to the speaker system. Such a transformation of low-frequencysignal (splitting and re-fusion), of course, causes irreversible distortion in it, due to the difference in frequency and dynamic properties of the two transistors of the circuit. This distortion reduces the sound quality at the output of the amplifier.
Push-pull amplifiers operating in class "A" do not reproduce complex audio signals well enough, since an increased constant current constantly flows in their arms. This leads to asymmetry of the half-waves of the signal, phase distortions and, ultimately, to the loss of sound intelligibility. When heated, two powerful transistors double the signal distortion in the low and infra-low frequencies. But still, the main advantage of the push-pull circuit is its acceptable efficiency and increased output power.
Push-pull transistor power amplifier circuit is shown in the figure.
This is a class "A" amplifier, but class "AB" and even "B" can also be used.
Transformerless Transistor Power Amplifier
Transformers, despite the progress in their miniaturization, are still the most bulky, heavy and expensive ERE. Therefore, a way was found to eliminate the transformer from the push-pull circuit by running it on two powerful complementary transistors of different types (n-p-n and p-n-p). Most modern power amplifiers use this principle and are designed to operate in class "B". A diagram of such a power amplifier is shown in the figure below.
Both of its transistors are connected according to a common collector (emitter follower) circuit. Therefore, the circuit transfers the input voltage to the output without amplification. If there is no input signal, then both transistors are on the border of the on state, but they are turned off.
When a harmonic signal is input, its positive half-wave opens TR1, but puts the p-n-p transistor TR2 in full cutoff mode. Thus, only the positive half-wave of the amplified current flows through the load. The negative half-wave of the input signal opens only TR2 and turns off TR1, so that the negative half-wave of amplified current is supplied to the load. As a result, a full power amplified (due to current amplification) sinusoidal signal is delivered to the load.
Single transistor amplifier
To assimilate the above, we will assemble a simple transistor amplifier with our own hands and figure out how it works.
As a load of a low-power transistor T of type BC107, we turn on headphones with a resistance of 2-3 kOhm, we apply the bias voltage to the base from a high-resistance resistor R of 1 MΩ, we turn on the decoupling electrolytic capacitor C with a capacity of 10 μF to 100 μF in the base circuit T. We will power the circuit from a battery of 4.5 V / 0.3 A.
If resistor R is not connected, then there is neither base current Ib nor collector current Ic. If the resistor is connected, then the voltage at the base rises to 0.7 V and a current Ib \u003d 4 μA flows through it. Coefficientthe current gain of the transistor is 250, which gives Ic=250Ib=1 mA.
Having assembled a simple transistor amplifier with our own hands, we can now test it. Connect the headphones and place your finger on point 1 of the diagram. You will hear a noise. Your body perceives the radiation of the mains at a frequency of 50 Hz. The noise you hear from the headphones is this radiation, only amplified by the transistor. Let us explain this process in more detail. An AC voltage of 50 Hz is connected to the base of the transistor through capacitor C. The voltage at the base is now equal to the sum of the DC bias voltage (approximately 0.7 V) coming from resistor R and the AC finger voltage. As a result, the collector current receives an alternating component with a frequency of 50 Hz. This alternating current is used to move the membrane of the speakers back and forth at the same frequency, which means we can hear a 50Hz tone at the output.
Hearing the 50 Hz noise level is not very interesting, so you can connect low-frequency sources (CD player or microphone) to points 1 and 2 and hear amplified speech or music.
