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A 1-watt audio amplifier trainer If you’re new to electronics, this 1-watt audio amplifi­er makes an ideal introduction. It’s easy to build & the compon­ent layout screen printed on top of the PC board is very similar to the circuit, to make signal tracing & voltage measurements easy. By JOHN CLARKE Audio amplifiers come in many shapes and sizes. They range from low-cost units with just enough power to drive a pair of headphones (eg, for a personal portable) right up to large units capable of driving the huge speaker blocks used at rock concerts. They are used in all sorts of equipment, including TV sets, CD players, stereo amplifiers, radio receivers and computer sound cards. Although building large amplifiers can be complicated, that certainly doesn’t apply to the low-power unit described here. This 1W Audio Amplifier Trainer is easy to assemble and uses only common, low-cost parts. If you accidentally damage any of these parts during construction, they can generally be replaced for less than 50 cents. To make it as easy for the beginner as possible, the PC board has a screen printed overlay (not included on our proto­type) which shows the positions of all the parts. This layout closely follows the circuit diagram layout, so that you can more easily understand how it works. To build the unit, all you have to do is follow the screen printed overlay. Provided your soldering is up to Performance With 12V Supply Output power into 8-ohm ................1.1W at onset of visible clipping Sensitivity........................................ 150mV for 1W output into 8-ohm Signal to noise ratio ������������������������ 74dB unweighted with respect to 1W, 20Hz to 20kHz bandwidth & 1kΩ input load; 101dB A-weighted Distortion......................................... <1.2% at 1kHz at 1W into 8-ohm Frequency response........................ -3dB at 60Hz & 90kHz (8-ohm load) 34  Silicon Chip scratch, your amplifier should work as soon as it is switched on. Output stage basics Fig.1 shows the complete circuit of our 1W Audio Amplifier Trainer. It employs what is known as a class AB “push-pull” or “complementary” output stage. These two terms have similar mean­ings and refer to the way in which the output transistors (Q3 & Q4) are connected. As shown in Fig.1, Q3 is an NPN transistor and Q4 is a PNP type (ie, they are complementary types). These two transistors have their emitters connected together via 1Ω resistors, while their collectors go to the supply rails (+9V in the case of Q3, ground or 0V in the case of Q4). In operation, Q3 conducts (ie, current flows from collector to emitter) when its base voltage is 0.6V higher than its emit­ter. Conversely, Q4 conducts when its base voltage is 0.6V lower than its emitter. To better understand this, take a look at Fig.2. This shows a simplified complementary output stage being 180k +11V 10 1M +6.3V INPUT GND Q1 BC548 C B B +6.7V E 0.1 VOLUME VR1 50k LOG +12V E +6.1V 1.5M B E QUIESCENT CURRENT VR2 200  +5.4V 100  47 Q2 BC558 C D1 1N4148 C +9-12VDC 470 16VW B E Q3 BC338 1 2.2k +6.1V 4mV 1  B 1k GND C 470 16VW 10  E Q4 BC328 0.1 C Fig.1: this 4-transistor circuit uses Q3 & Q4 as comple­mentary emitter followers (having close to unity gain) and Q1 & Q2 as the voltage gain stages. Because the output of the amplifi­er is at half the supply, a DC blocking capacitor is required to couple the amplifier to the loudspeaker. LOUDSPEAKER 8 VIEWED FROM BELOW 1W AUDIO AMPLIFIER TRAINER driven by a sinew­ave signal. During the positive (top) half-cycle of the input waveform, the top transistor conducts and the bottom transistor remains off. Then, during the negative half-cycle of the input signal, the top transistor turns off and the bottom transistor conducts. The amplified signal appears at the commoned emitters of the two transistors. Crossover distortion If you look closely at the output waveform shown in Fig.2, you can see that it doesn’t look the same as the input – there’s a small “step” in the waveform each time it crosses the 0V line. We call this effect “crossover distortion”. It occurs because the input signal must rise to +0.6V before the top transistor begins to conduct and Facing page: the prototype of our 1W Audio Amplifier Trainer. Kits will be supplied with a screen printed overlay on the PC board. must drop to -0.6V before the bottom transistor begins to conduct. For input signal voltages between ±0.6V, both transistors are off and so there is effectively no signal output over this range. This means that the amplified output signal is distorted at the crossover points, as the input signal swings from +0.6V to -0.6V. To reduce this distortion, we have to apply a permanent 0.6V bias to both transistors, so that they are always slightly on, regardless of the input signal. This simply involves separat­ ing the bases of the output pair and connecting them instead to network with 1.2V across it (0.6V for each transistor). What happens now is that the top transistor will immediate­ly conduct as soon as the input signal rises above 0V. Similarly, the bottom transistor will conduct as soon as the input signal drops below 0V. As a result, most of the crossover distortion is eliminated and the sound quality is greatly improved. This type of output stage biasing is referred to as “class AB”. That’s because it operates mainly as a class B output stage, where each transistor is completely off for half the input cycle, but is also biased slightly towards the class A condition, in which the output devices are always biased on. Clipping Crossover distortion is not the only form of distortion that can occur in audio amplifier stages. Another major source of distortion is known as “clipping”. This occurs when an amplifier is driven into overload. If you go back to Fig.2, you can see that while there is crossover distortion, the peaks of the output waveform still follow the input signal. This means that the transistors can handle the input signal without overloading. But what happens if the input signal becomes too large to handle? In a perfect amplifier, the output signal could swing as far as the positive and negative supply rails. In practice, however, the maximum output voltage swing is somewhat less V+ NPN INPUT OUTPUT 0V PNP 0V CLIPPING V- Fig.2: a complementary emitter follower output stage operating in class-B (ie, no bias) will produce crossover distortion in the waveform. A small bias on the output transistors will eliminate most of this distortion. Fig.3: all amplifiers can be driven into clipping if the input signal is too large. An amplifier should be biased so that clip­ping is symmetrical (ie, the same degree of clipping at top and bottom) so that power output before the onset of clipping is maximised. June 1995  35 180k 10uF 1M SIGNAL INPUT Q3 BC338 Q2 BC558 PARTS LIST 470uF +9-12V Q1 BC548 0V D1 VR1 1 0.1 470uF 2.2k 100 1.5M 1 VR2 10  TO LOUDSPEAKER Q4 BC328 GROUND 0.1 47uF 1k Fig.4: the screen print overlay for the 1W Audio Amplifier Train­er PC board. Compare this layout with the circuit of Fig.2. than this, due to the voltage losses across the output devices and their emitter resistors. Because of this, a large input signal can easily overload the output stage. This is called “clipping” and its effect on the output waveform is shown in Fig.3. As can be seen, the positive and negative peaks of the waveform are flattened, resulting in severe distortion of the audio. On normal program material, a small amount of clipping may not be audible but in severe cases, it sounds horrible. Another thing that emerges from Fig.3 is that the DC output of the amplifier should sit at about half supply under no-signal conditions. That way, the output can swing equally to the posi­ tive and negative supply rails when an input signal is applied, thus reducing the chances of clipping. On the other hand, if the DC output is set too high, then the positive signal peaks will not have as far to swing as the negative peaks before they are ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 1 2 36  Silicon Chip Value 1.5MΩ 1MΩ 180kΩ 2.2kΩ 1kΩ 100Ω 10Ω 1Ω clipped. The reverse also applies. This gives rise to an effect known as asymmetrical clipping and is highly undesirable since it effectively reduces the available power output. By the way, Fig.2 shows a transistor output stage with positive and negative supply rails and the output referenced to 0V; ie, halfway between the two supply rails. That is how the more powerful amplifiers are designed but low power amplifiers such as the one discussed here usually have a single supply rail and the DC output is set at close to half this supply voltage. Because of this, a DC blocking capacitor is required between the output transistor emitters and the loudspeaker load. If the capacitor was not includ­ed, a heavy DC current would flow through the speaker, even with no signal applied and this could burn out the speaker or damage the amplifier’s output transistors. One thing we haven’t mentioned so far is that the output stage provides RESISTOR COLOUR CODES 4-Band Code (1%) brown green green brown brown black green brown brown grey yellow brown red red red brown brown black red brown brown black brown brown brown black black brown brown black gold gold 1 PC board, code 01306951, 109 x 77mm, with screened overlay 1 50kΩ (log) PC mount potentiometer (VR1) 1 200Ω miniature vertical trimpot (VR2) 6 PC stakes 4 rubber feet 1 9V battery 1 battery clip 1 miniature 8-ohm loudspeaker Semiconductors 1 BC548 NPN transistor (Q1) 1 BC558 PNP transistor (Q2) 1 BC338 NPN transistor (Q3) 1 BC328 PNP transistor (Q4) 1 1N4148 signal diode (D1) Capacitors 2 470µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.1µF MKT polyester Resistors (0.25W, 1%) 1 1.5MΩ 1 1kΩ 1 1MΩ 1 100Ω 1 180kΩ 1 10Ω 1 2.2kΩ 2 1Ω only current amplification. However, an audio amplifi­er also needs a voltage amplification stage (or stages) to boost the input voltage so that it’s enough to drive a loudspeaker. This is the job of transistors Q1 and Q2 in the circuit of Fig.1. Circuit details In addition to the transistors, we need only a handful of parts to produce a complete working amplifier. The input signal is initially applied to potentiometer VR1 which functions as the volume control. The output 5-Band Code (1%) brown green black yellow brown brown black black yellow brown brown grey black orange brown red red black brown brown brown black black brown brown brown black black black brown brown black black gold brown brown black black silver brown 180k Q2 1M 10uF Q3 SIGNAL INPUT Q1 1 470uF 10  VR2 1 100  2.2k 1.5M +9-12V 0V D1 0.1 VOLUME VR1 470uF Fig.5: almost identical to Fig.4, this is the component overlay for the PC board. It also shows the copper pattern. TO LOUDSPEAKER Q4 0.1 GROUND 47uF 1k from VR1’s wiper is fed to the base of transistor Q1 via the 0.1µF coupling capacitor. This coupling capacitor is necessary because it prevents the DC voltage at the base of Q1 from being varied by different settings of VR1. Be­cause the bias on Q1 determines the DC voltage at the output of the amplifier, we don’t want it varied each time you change the setting of the volume control. Q1 is connected as a common emitter stage and is biased to just over half supply using the 1MΩ and 1.5MΩ resistors at its base. It varies its collector current in response to the audio signal applied to its base and, in turn, drives the base of PNP transistor Q2. Note that because Q2’s base is driven by Q1’s collector, the audio signal is inverted at this point. Q2 is also connected as a common emitter stage and provides most of the voltage gain of the amplifier. Its collector current flows partly into the bases of the output transistors (Q3 and Q4), while the rest goes through the 1kΩ resistor and 8Ω loud­speaker to ground (0V). The two output transistors, Q3 and Q4, are connected as complementary emitter followers. They are slightly biased into forward conduction by the voltage developed across diode D1 and trimpot VR2. This trimpot allows the forward bias voltage applied to the output pair (and thus their quiescent current) to be adjusted to minimise the crossover distortion. Diode D1 is included to provide a measure of temperature compensation for the bias network. As the ambient temperature increases, the voltage across it reduces and this partly compen­sates for the similar reduction in Vbe voltage of Q3 and Q4, as they warm up. The 1Ω emitter resistors apply a small amount of local negative feedback to Q3 and Q4 and this also helps stabilise the quiescent current. By the way, the term “quiescent current” refers to the current drawn by the amplifier when no signal is present. Quiescent current is often referred to as “no signal” current. As soon as signal is applied to the amplifier, more current is drawn. Negative feedback & stability The 2.2kΩ resistor connected between the output of the amplifier and the emitter of Q1 forms the negative feedback path. This resistor, together with Q1’s 100Ω emitter resistor, sets the AC voltage gain of the amplifier to 23. The associated 47µF capacitor rolls off the bass response below 34Hz. Note the network consisting of a 10Ω resistor and a 0.1µF capacitor connected across the amplifier’s output. Often referred to as a Zobel network, this network helps ensure that the ampli­fier does not tend to oscillate supersonically when it has no load or when its effective output load becomes a very high value, as it can at high frequencies due to the inductance of a loud­speaker. Bootstrapping A point to note is that the 1kΩ resis- tor in the collector load for Q2 is not connected directly to ground. Instead, it goes to ground via the loudspeaker. To understand why this has been done, it is important to note that the output transistors func­tion as emitter followers and thus have almost unity gain. This means that there is almost no difference in AC signal voltage between Q2’s collector and the output to the loudspeaker, and so there is very little AC voltage drop across the 1kΩ resistor. As a result, Q2’s collector “sees” a much higher AC im­pedance than the nominal 1kΩ load connected. It is therefore able to provide more drive to the output stage and operate with less distortion than would otherwise be the case (eg, if the 1kΩ resistor was connected directly to ground). This technique is called “boot­ strapping” and is commonly used in amplifiers to improve the linearity. However, this simple form of boot­strapping is not used in higher performance amplifiers as it has a serious drawback – if you disconnect the loudspeaker, the 1kΩ resistor has nowhere to go. Thus, the bases of the output transistors are pulled up to the positive supply and the amplifi­er latches up. This can be a trap for young players because if you try to make voltage measurements on the circuit without a load connect­ed, the circuit won’t work! Power for the circuit can be derived from any 9-12V DC source capable of supplying up to 100mA (eg, batteries June 1995  37 Fig.6: this is the full-size artwork for the PC board. or a 9V DC plugpack). A 470µF electrolytic capacitor provides supply line filtering, while a 180kΩ resistor and 10µF capacitor provide further supply line decoupling for the bias network connected to Q1. This prevents the output from following any changes to the supply voltage. Construction The 1W Audio Amplifier Trainer is constructed on a PC board coded 01306951 and measuring 110 x 78mm. It features a screen printed component overlay on the top side which is very similar to the circuit diagram, as noted above. The screen pattern dia­ gram is shown in Fig.4 while the almost identical component overlay diagram is shown in Fig.5. Most of the components will go on the board as shown with two ex- ceptions. Transistors Q1 and Q3 will need to have their base leads (centre lead) bent between the other leads to match the holes in the PC board. This is easily accomplished with a pair of pliers. We used PC stakes for the 9-12V and 0V supply inputs, the loudspeaker outputs and the signal input terminals IN and GND. Use the colour code chart to check each resistor as it is in­stalled. If you are not sure of the values, measure each resistor with your multimeter. The electrolytic capacitors must be mounted with the cor­rect polarity so that the positive marking on the overlay corre­sponds to the positive lead on the component. Note that while 16VW electrolytics are specified in the circuit, you may be supplied with 25VW or 35VW ca- pacitors instead. These will be a little larger but will work just as well. When installing the transistors, be sure to get each one in its correct place otherwise they may be damaged. Make sure that the diode is inserted the correct way around, too. When all the parts have been installed correctly and sol­dered in place, check your work again to be sure everything is correct. Now set VR1 fully clockwise. This will minimise the current through Q3 and Q4 when power is applied. You can now connect up a loudspeaker and apply power. You can use a 12V battery, 12V power supply or a 9-12V DC plug­pack. The voltage measurements on the circuit were taken with the supply voltage set to exactly 12V. Connect a multimeter across one of the 1Ω resistors and set the multimeter to read DC mV. Apply power and set VR1 for a reading of around 4mV. This will set the quiescent current through Q3 and Q4 at 4mA. Now check the other voltages on the circuit to see that they are within 10% of those shown. If they differ widely, you have a problem. Note that if you use a digital multimeter to measure the voltage at the base of Q1, the value will be loaded slightly by the 10MΩ input impedance of the meter. On other hand, if you use an older analog multimeter to measure this base voltage, its sensitivity is likely to be “20,000 ohms per volt” and thus its loading when set to the a 10V DC range, for example, will be only 200kΩ. This would seriously load down the base of Q1 and thus lead to a wildly SC inaccurate voltage reading. 20 Electronic Projects For Cars On sale now at selected newsagents Or order your copy from Silicon Chip. Price: $8.95 (plus $3 for postage). Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 38  Silicon Chip