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200W/350W Mosfet Amplifier Module Here is a rugged amplifier module which will deliver 200 watts RMS into 8Ω loads and 350 watts RMS into 4Ω loads. It uses eight Toshiba plastic encapsulated Mosfets which are each rated at 200 volts, 12 amps & 150 watts. Design by ANTHONY HOLTON When it comes to making complementary Mosfets suitable for big audio power amplifiers, two Japanese companies, Hitachi and Toshiba, have the game sown up. Many enthusiasts will have had experience with Hitachi TO-3 metal encapsulated Mosfets and their recent plastic TO-3P replacements but these plastic devices from Toshiba are something else and have much higher ratings: VDSS 200V (drain-source voltage), drain current 14  Silicon Chip 12 amps and power dissipation 150 watts. The Toshiba devices used are 2SK1530 for the N-channel devices and 2SJ201 for the P-channel devices. They are physically much larger than then familiar Hitachi plastic devices (eg, 2SK1037 and 2SJ161). Compared with the TO-3P encapsulation which is 15mm wide and 20mm high, the Toshiba TO-247 devices measure 20mm wide and 26mm high, not counting the lead dimensions. A particular advantage of the Toshiba devices is that the drain is connected to the heatsink tab which means that the capacitance between tab and heatsink has no practical effect on the performance (ie, it cannot lead to high frequency instabili­ty). Anthony Holton has come up with a design that delivers the goods in terms of power output and with eight devices employed, it should be rugged and reliable. The basic module is a PC board measuring 200 x 90mm with the eight Mosfets mounted along one edge on an aluminium right-angle bracket. Six 5W wirewound source resistors are mounted under­neath the PC board. If your are going to build this module, you will need a transformer with a power rating of at least 500 watts, together with a substantial rectifier and filter capacitors. These will need to be mounted in a roomy chassis with +70V 4.7k 10k Q4 BC546 C B E Q6 MJE350 4.7k Q5 BC546 C B 10pF B 100  E E C C 10 BP 220  B B 470  C C E E Q13 C B A LED1 ORN Q3 BC546 Q11 470   ZD3 15V 15k ZD4 15V 470  330 0.33 5W 0.33 5W 470  G Q12 S 0.47 10  0.22 18k B 10k E C VIEWED FROM BELOW PLASTIC SIDE ZD1 18V Q1 BD681 B 18k 22k 1W 470  470  470  Q7 MJE340 C 100  B E C E 0.33 5W D G Q14 S 0.33 5W 0.33 5W 0.33 5W 4x2SJ201 D G S G 0.33 5W 0.33 5W Q16 S D G Q18 S D Q9 MJE340 100  100 160VW D 4x 2SK1530 D D S G D S G S G K B 470  Q10 MJE340 E .0012 15k Q17 470  10pF Q15 18pF INPUT F1 5A E ZD2 15V BIAS VR1 10k Q2 BC546 Q8 MJE350 47 160VW F2 5A 100 160VW -70V 47 160VW GDS E C B A K 200W/350W MOSFET AMPLIFIER Fig.1: the circuit is fairly conventional with a differential input amplifier, Q2 & Q3, driving cascode transistors Q4 & Q5. These drive the voltage amplifier which consists of a differen­tial pair Q6 & Q8, loaded by a current mirror, Q7 & Q9. The voltage amplifier, in turn, drives the Mosfet output stages (Q11-Q18). a very substantial heatsink. The overall cost is not likely to leave much change out of $500. After all, this is a big power amplifier we’re talking about and they don’t come cheap. Circuit details Now let’s have a look at the circuit – see Fig.1. The circuit is fairly conventional with a differential input amplifier, Q2 & Q3, driving cascode transistors Q4 & Q5. These drive the voltage amplifier which consists of a differen­tial pair Q6 & Q8, loaded by a current mirror, Q7 and Q9. The voltage amplifier drives the Mosfet output stages which has all devices connected in source-follower mode to give a large current gain. That summarises the circuit but let’s look at it in more detail. The in- put signal is fed in via a 10µF bipolar electroly­tic capacitor. The 10µF capacitor and the associated 15kΩ bias resistor form a high pass filter which sets the -3dB low frequency response to 1Hz. The input signal also passes via a 220Ω resistor and is shunted by a .0012µF capacitor which together form a low-pass filter to limit frequencies above 600kHz. The input differential amplifier is operated in cascode mode, as noted above. NPN transistors Q2 and Q3 are the differen­ tial pair and their collectors drive the emitters of the cascode transistors Q4 and Q5 and these improve the linearity and fre­ quency response of the stage but this is not the main reason for using the cascode connection. Note that the positive supply of the amplifier is +70V, too high for the 65V collector rating of Q2 and Q3 which are BC546 low noise types. The 15V zener diode ZD2 acts as a voltage refer­ence for the cascode transistors Q4 and Q5 and thus their emit­ters sit at around +14.4V, well within the collector rating of Q4 and Q5. Hence, the cascode transistors act to “regulate” the voltage for the differential pair. Because the cascode transistors Q4 & Q5 have their bases tied to a voltage source, they are effectively in “common base” mode. Hence, as already noted, their input signals appear at their emitters and the outputs at the collectors, to drive the following voltage amplifier stage consisting of differential transistors Q6 & Q8. Voltage amplifier stage The emitters of Q2 & Q3 are connected to a current source comprising transistor Q1 and zener diode ZD1. This is the “tail” of the so-called long tailed pair”. Zener diode ZD1 June 1994  15 This photograph shows how the Vbe multiplier transistor (Q10) is mounted on the top of Mosfet Q11 (metal side down). Smear the metal surface of Q10 with heatsink compound before bolting it into position. sets a constant voltage at the base of Q1 which then applies about 17V to its 18kΩ emitter resistor. This sets the current through Q1 at just under 1mA and this is then shared as emitter current by the input transistors Q2 and Q3. The constant current source needs to withstand almost the full 70V of the negative supply rail and this is why a BD681 is specified. It happens to be a Darlington transistor but more importantly, its collector voltage rating is 100V. As noted above, the voltage amplifier stage is another differential stage but with current mirror loading. Q6 & Q8 are the differential transistors and these are loaded by the current mirror, Q7 & Q9. The current mirror is really another form of constant current load. In effect, NPN transistor Q7 is connected as a forward biased diode and this provides a reference voltage to the base of Q9 which then acts as a constant current load for the collector of Q8. The term “current mirror” comes from the current sharing action in the differential pair. If there is any tendency for Q8 to draw more current then the other half of the differential pair, Q6 is forced to draw less current. The smaller collector current then reduces the voltage applied by Q7 to the base of Q9. Q9 is then throttled BR1 MDA3504 A 50V 240VAC +70V 50V N 10000 75VW 0V E CHASSIS 16  Silicon Chip 10000 75VW -70V Fig.2: the suggested power supply circuit for the amplifier module. Note that the rectifier bridge will dissipate a fair amount of power & this should be taken care of by bolting it to the chassis or to a large heatsink. back to restore the original current condi­ tions. The result of using the current mirror connection is a high gain and excellent linearity. Current mirror stages are commonly found in integrated circuit op amps. Mosfet output stages The complementary output stage comprising the eight Mosfets is biased into class AB operation by the Vbe multiplier transis­tor, Q15, together with an orange light emitting diode, LED 1. This is the quiescent current setting and in this amplifier it is 100mA per device or a total of 400mA. In effect, a standard Vbe multiplier has a bias voltage applied by a trimpot (in this case VR1) between its base and emitter and it amplifies this voltage so that the total voltage appearing between its base and collector is the product of Vbe (the base-emitter voltage) and the ratio of the total resistance of the trimpot to the resistance between base and emitter. To give an example of how this works, let’s say that the 10kΩ trimpot was set so that its resistance between the transistor base and emitter was 2kΩ and the resultant Vbe was 0.6V. The total voltage between collec­ tor and emitter would then be (0.6V x 10kΩ/2kΩ) = 3V. The Vbe multiplier transistor is Q11 4.7k 4.7k +70V 10k 47uF F1 0.47 Q5 Q4 Q2 ZD2 .0012 Q9 B C E 22k 1W 0.22 B C E Q1 10  330uF 220  0. 33  0. 33  Q18 470  ZD4 470  LED1 470  A 470  18pF VR1 Q3 470  18k 15k 0. 33  RESI ST ORS MOU NTED ON COPPER SI DE O F B OARD 100  100  Q7 100uF ZD1 18k 10k 0V,SPKR- 47uF -70V 10uF BP F2 GND INPUT Fig.3: the parts layout on the PC board. Note that the Mosfet power transistors (Q11-Q18) must be isolated from the heatsink using silicon impregnated rubber washers & isolating bushes. The 0.33Ω resistors (shown dotted) are mounted on the copper side of the board. Take care with component orientation. attached to the same heat­sink as the output transistors so if they heat up, the Vbe multi­plier’s voltage is automatically reduced to compensate. Hence the quiescent current stays pretty constant and thermal runaway is avoided. This scheme works well for amplifiers with bipolar transistors and is not necessary in those which used Hitachi Mosfets in the past. However, the thermal characteristics of these Toshiba Mosfets is such that quiescent current stabilisa­tion with a Vbe multiplier transistor is necessary. The catch is that the standard Vbe multiplier circuit overcompensates. This means that when the amplifier is delivering lots of power and is getting Q16 0. 33  0. 33  0. 33  10pF ZD3 B C E Q6 Q14 SPKR+ 15k 100uF 470  10pF Q8 B C E 100  Q12 Q10 0. 33  470  470  470  Q13 0. 33  Q15 0. 33  Q17 hot, the Vbe multiplier reduces its voltage to the point that no forward bias is applied to the output stage. In other words, it reverts to pure class B opera­tion when it gets hot and distortion rises to high levels. The cure is to modify the Vbe multiplier so that it applies less compensation. This is achieved by connecting LED 1 into the emitter circuit of Q15. The result is a circuit which still overcompensates to some extent but this affords a higher degree of thermal stability and prevents damage to the amplifier. Overdrive protection 15V 1W zener diodes ZD3 & ZD4 are connected between the commoned gate and source connections of the complementary Mos­ fets. They are included to prevent the occurrence of gross gate drive which could result if the output of the amplifier was shorted. The zener diodes prevent gate damage but do not provide any protection against excessive current in the output stage; that is provided by the fuses in the positive and negative supply lines. Note that 470Ω resistors are connected in series with the gates of each Mosfet. These provide some limiting of the frequen­cy response and thus reduce the possibility of parasitic oscilla­tion. Each Mosfet also has a 0.33Ω source resistor and these provide local degeneration (current feedback) to slightly improve thermal stability and help promote current sharing amongst the output devices. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 2 2 2 9 1 3 1 8 Value 22kΩ 18kΩ 15kΩ 10kΩ 4.7kΩ 470Ω 220Ω 100Ω 10Ω 0.33Ω 5W 4-Band Code (1%) red red orange brown brown grey orange brown brown green orange brown brown black orange brown yellow violet red brown yellow violet brown brown red red brown brown brown black brown brown brown black black brown not applicable 5-Band Code (1%) red red black red brown brown grey black red brown brown green black red brown brown black black red brown yellow violet black brown brown yellow violet black black brown red red black black brown brown black black black brown brown black black gold brown not applicable June 1994  17 The completed amplifier board should be bolted to a large finned heatsink with a rating of at least 0.5-0.7 degrees/watt. Don't skimp on the heatsink, otherwise the amplifier will be unable to deliver its rated power. Three capacitors are included in the circuit to roll off the open loop high frequency gain and hence ensure stability. They are the 10pF capacitors between the bases and collectors of Q6 & Q8 and the 18pF capacitor between the collector of Q9 and the base of Q3. The overall AC voltage gain of the amplifier is set by the 15kΩ and 470Ω feedback resistors connected to the base of Q3. These set the gain to 33 times. The resulting input sensitivity is 1.2V RMS for 200 watts into 8Ω or 1.13V RMS for 350 watts into 4Ω. Power supply To run this module, you will need a big power supply. If you want full power into an 8-ohm load you will require a 300VA transformer with two 50V windings. If you want full power into a 4-ohm load, you will need a 600VA transformer. In prac­tice, the only readily available transformer which is suitable is a 500VA toroid available from Altronics (Cat M-3140). The circuit of a suggested power supply is shown in Fig.2. Note that the rectifier bridge will itself dissipate a fair amount of power and this should be taken care of by bolting it to the chassis or heatsink. The filter capacitors should be a minimum of 10,000µF 75VW but preferably should be a bank of 20,000µF or more, for each supply rail. Don’t skimp on the power supply otherwise you will reduce the available performance. For the purpose of this article we shall assume that you have the power supply and chassis details organised to your satisfaction. Where to buy the kit Assembly This design will be available in kit form from Computer & Electronics Services Pty Ltd who own the copyright on the PC board. The kit in­cludes all parts, the aluminium mounting bracket and the PC board which is made from two ounce copper and is tinned, solder masked and silk screened. Price is $159 plus $8.00 for postage and handling within Australia. Fully built and tested modules are $199.00 plus $8.00 postage and handling. Payment may be made by cheque, money order, Bankcard, Visa­card or Mastercard. Send remittances to Computer & Electronic Services Pty Ltd, 27 Osborne Avenue, Launceston, Tasmania 7250. Phone (003) 34 4218. Fax (003) 31 4328. Before you begin any soldering of the PC board, check the copper pattern thoroughly for any shorts or breaks in the copper tracks. The board for the kits will be supplied with a screened component overlay on the top and a green solder mask underneath. The component wiring diagram for the PC board is shown in Fig.3. You can start by inserting and soldering all the PC stakes, resistors, fuse- 18  Silicon Chip Performance of Prototype Output power......................... 200W into 8 ohms, 350 watts into 4 ohms Frequency response ............. 4Hz to 56kHz at -3dB points Input sensitivity ..................... 1.2V RMS (for 200W into 8 ohms) Harmonic distortion .............. <.07% from 20Hz to 10kHz, typically <.005% Signal to noise ratio ������������� -122dB unweighted (20Hz to 20kHz); -126dB A-weighted Damping factor ..................... >200 (for 8 ohm loads) Stability ................................. unconditional holders, the small capacitors and the multi-turn trimpot in their respective positions. Leave the 5W wire­wound resistors on the copper side of the PC board for the time being – we’ll come to these later on. Next, insert the electrolytic capacitors, then continue by inserting the smaller semiconductors such as the BC546s, MJE340s, MJE350s, BD681, zener diodes and the LED. Do not mount the MJE340 for the Vbe multiplier (Q10) yet as this is mounted on one of the Mosfets. Next mount all of the Mosfets on the aluminium angle brack­et and PC board. The leads of each Mosfet will need to be bent at 90° so that they go through the relevant holes in the PC board. The Mosfets should be mounted using silicon impregnated rubber washers and isolating bushes and secured with M3 bolts and nuts. Do not solder them in at this stage. After mounting the Mosfets on the heatsink bracket and PC board, test with a multimeter to check that they are all isolat­ed. Set the multimeter to a high Ohms range and test for an open circuit between the metal bracket and the drain lead of each device. If a short circuit is detected, unbolt the offending device and check for a misplaced washer or bush or metal burrs around the mounting hole. Once satisfied that there are no shorts on any of the devices, solder the Mosfets in place. The next task is the mount­ing of the Vbe multiplier transistor (Q10). This is mounted on top of Q11 with the metal tab facing down and using the existing mounting bolt. Once it is mounted, trim the leads of Q10 back to about 10mm long and tin them with solder. Cut three lengths of hookup wire (40mm each) and strip and tin wires at both ends. Insert and solder the three wires in the three remaining holes in the PC board, adjacent to trimpot VR1. Solder each wire to the appropriate base, emitter and col­lector leads of Q10. The last task is the mounting of the 5W wirewound source resistors on the copper side of the PC board. Cut each lead on these resistors to a length of 12mm and then bend them down at 90°. This done, bend a small flat hook at the end of each lead and then solder them in the appropriate positions as shown by the compon­ent overlay. Testing The module should be bolted to a large heatsink with a rating of at least 0.5-0.7°/watt. Remove the fuses and solder a 22Ω 5W resistor in their places. These resistors provide a convenient way of setting and measuring the quiescent current and also protect the amplifier in the event that there is a fault. They may go up in smoke but the amplifier will be protect­ed. Measure the resistance (set your multimeter to the Ohms range) between base and collector of Q10. Adjust VR1 so that this resistance is zero. This adjustment ensures that when power is first applied to the module, the output stage is biased off. Make the appropriate supply and ground connections to the power module. Now apply power and check the DC voltage at the output of the amplifier. It should be within ±50mV of 0V. Now connect the multimeter across one of the 22Ω 5W resis­tors on the fuseholders. The DC voltage should be zero. Now adjust trimpot VR1 so that the voltage across the 22Ω resistors is 13.2 volts. This is equivalent to a total quiescent current in the output stage of 400mA or 100mA per device. PARTS LIST 1 PC board 1 aluminium extrusion, 200 x 90mm x 6mm (see text) 1 large heatsink, Jaycar Cat HH8594 or equivalent 2 M205 PC mount fuseholders 2 5A or 10A M205 fuses 1 10kΩ multi-turn trimpot 6 PC stakes Semiconductors 1 BD681 NPN Darlington transistor (Q1) 4 BC546 NPN low noise transistors (Q2, Q3, Q4, Q5) 2 MJE350 PNP power transistors (Q6,Q8) 3 MJE340 NPN power transistors (Q7, Q9, Q10) 4 2SK1530 N-channel Mosfets (Q11, Q13, Q15, Q17) 4 2SJ201 P-channel Mosfets (Q12, Q14, Q16, Q18) 3 15V 1W zener diodes (ZD2, ZD3, ZD4) 1 18V 1W zener diode (ZD1) 1 orange LED (LED1) Capacitors 1 330µF 16VW electrolytic 2 100µF 160VW PC electrolytics 2 47µF 160VW PC electrolytic 1 10µF 50VW bipolar electrolytic 1 0.47µF 100VW MKT polyester 1 .22µF 100VW MKT polyester 1 .0012µF 100VW MKT polyester 1 18pF ceramic 2 10pF ceramic Resistors (0.25W, 1%) 1 22kΩ 1W 9 470Ω 2 18kΩ 1 220Ω 2 15kΩ 3 100Ω 2 10kΩ 1 10Ω 2 4.7kΩ 8 0.33Ω 5W 2 22Ω 5W (for setup & testing) You can check this by measuring the voltage drop across any of the 0.33Ω 5W source resistors mounted on the copper side of the board. This will be 33mV but will vary over a fair range for each device, due to variations in the forward transfer admit­tance. Now remove the 22Ω resistors across the fuseholders and replace the fuses. Use 5A fuses if you are using an 8-ohm load and 10A fuses for a SC 4-ohm load. June 1994  19