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A look at the TDA7377 quad 12V amplifier IC The TDA7377 IC from ST Microelectronics is the main component of this month’s 12V Mini Stereo Amplifier. It’s not a new chip – they’ve been making them since at least 1998 – but it is the first time we’ve used it so it deserves some elaboration. It comes in a 15-pin “Multiwatt” package similar to TO-218 and is available in both horizontal and vertical mounting packages. By NICHOLAS VINEN T HIS IC is designed for use in car stereo systems and can provide four single-ended channels, two bridged channels or a combination of two single-ended and one bridged channel. Maximum power depends on speaker impedance, supply voltage and channel configuration but the most useful figures are 4 x 10W into 2Ω, 4 x 6W into 4Ω and 2 x 20W into 4Ω. Noise performance and channel separation are also quite good. The S/N ratio is typically close to -100dB and channel separation is generally at least 60dB at 10kHz. This is surprisingly good when you consider that all four power amplifiers share the same package and power supply pins. The best features of this IC are its low distortion (down to 0.02% or less) and high power. The most basic circuit for driving two speakers requires just the IC, five small capacitors, one large capacitor (for supply bypassing) and one resistor. It doesn’t get much easier than that! The TDA7377 quad amplifier comes in a 15-pin Multiwatt package. Because there are no external gainsetting resistors, this means that the gain is internally fixed. This is both a blessing and a drawback – while it reduces the component count, we can’t adjust the gain to our liking. However, their choice of 20dB gain per circuit is reasonable. This actually results in 26dB of gain in bridge mode. The reason is that in bridge mode, twice the voltage is placed across the speaker as in singleended mode. This equates to +6dB of additional gain. As is typical for integrated amplifiers, there is a standby pin which allows the amplifiers to be electronically shut down when not in use. In this condition, the quiescent current is around 1µA. The standby pin also prevents clicks and pops during turnon and turn-off, because it either mutes or un-mutes the signal paths when it is switched. Protection The maximum supply voltage for the IC is 18V but it can withstand up to 28V when it is not operating and spikes of up to 50V for no longer than 50ms. Each channel can deliver up to 3.5A continuously (4.5A peak) and the maximum dissipation is 36W. In fact, not only can the IC handle voltage spikes but it is virtually indestructible if kept within its limits. Output shorts, excessive current, overheating, inductive and capacitative loads, short-term open-circuit ground wiring, reversed battery – none of these will destroy it, thanks to internal protection circuitry. The thermal limiting isn’t just a simple cut-out which disables the amplifier either. The current limiting gradually increases with die temperature, so that at first it creates only mild output distortion while reducing the dissipation in an attempt to prevent further temperature increases. If driven hard enough it will eventually lead to heavy clipping but this is a nice feature. The amplifier can still be used if it is approaching its junction limits and if the overload is temporary or marginal, the listener may not even notice. Implementation While implementing an amplifier with this IC is simple, there are a few tricks. Firstly, because it is optimised for bridge configurations, two of the amplifier circuits are inverting and two are not. This means that if you want 20  Silicon Chip siliconchip.com.au Vcc B Vcc C Q1 DRIVER E (NPN) B B + Vbias B C Q3 POWER E (NPN) + Q1 DRIVER E (NPN) Q3 POWER C (PNP) Vbias – – IN OUT IN OUT + + Vbias Vbias B – E B C E Q4 POWER C (PNP) – Q2 DRIVER (PNP) B Fig.1: the traditional amplifier output stage consists of two complementary Darlington transistor pairs in emitter-follower configuration. to use them as four separate channels, you need to reverse the speaker wires for the two which are being driven from the inverting amplifiers. That way, all four outputs are kept in phase. Care must also be exercised to keep the power ground and signal ground lines separate, except where they meet at the star-earth point. The purpose of the “SVR” capacitor is not explained in the data sheet but “SVR” stands for “Supply Voltage Rejection”. This capacitor filters the internal half supply in the IC, so that supply variations do not couple into the signal paths. This is why it must be connected to the signal ground. If connected correctly, the supply voltage rejection figure is in excess of 50dB at 300Hz. One feature that we did not use in our 12V Minis Stereo Amplifier design is the diagnostic pin. It is an open collector output which is turned on during clipping, thermal limiting or an output short circuit. It can be used to light an indicator lamp or drive some kind of fault display. Alternatively, a circuit can be added to engage dynamic range compression if high volume is causing the outputs to clip. Clipping can be distinguished from other faults by noting the duration of the diagnostic output pulses or by measuring the average current sunk E B C Q2 DRIVER (PNP) DARLINGTON OUTPUT STAGE siliconchip.com.au C E C Q4 POWER E (NPN) COMPOUND OUTPUT STAGE Fig.2: the compound pair output stage configuration. It’s advantage is that is has a greater voltage swing than the Darlington arrangement shown in Fig.1. by that pin. Shorter pulses indicate clipping, longer pulses are caused by short circuits or thermal limiting. Output stage The most interesting feature of the IC is its output stage. It achieves a true rail-to-rail swing (minus transistor saturation at high currents) with no possibility of oscillation and yet doesn’t introduce high levels of distortion. Let’s see how they did it. Integrated amplifiers like the TDA­ 7377 are sometimes referred to as “power op amps”. The main difference between an amplifier IC and an op amp is the amount of current they can deliver. The TDA7377 can be likened to a high power rail-to-rail op amp. There are two different types of railto-rail op amps. The first is usually referred to as just “rail-to-rail” or “RR” and this means that the output voltage swing goes very close to both the positive and negative supply. How close depends on the load – at light loads (ie, high impedances) it will swing very close indeed, often to within a few millivolts. At heavier loads (ie, low impedances) it may only go within a half a volt or so, due to resistance effects in the output transistors. The second type is usually more expensive and is called “rail-to-rail input/output” or “RRIO”. This means that not only can the output voltage go close to both supply rails but the input common mode voltage range also extends to, or beyond, both rails. Since in this case we are dealing with a power amplifier that has a large fixed voltage gain, the inputs do not need to extend to the rails. With a gain of 20dB (a factor of 10), a 1.2V peakto-peak sinewave input signal (424mV RMS) is enough to drive the outputs to a full 12V swing. So RRIO is not really necessary for an AC signal when there is enough voltage gain. Traditional output architectures A traditional amplifier output stage consists of two complementary Darlington transistor pairs in an emitterfollower configuration – see Fig.1. This is very simplified but shows the most important components. This output stage can only swing to within about 1.4V of each supply rail, because of the two base-emitter drops in each transistor pair. In other words, if VCC is 12V and the base of Q1 is at 12V, the emitter of Q3 will be around 10.6V. If we used this architecture for a 12V amplifier, the maximum output swing would be 9.2V peak-to-peak, resulting in a poor maximum power figure. Fig.2 shows a similar but arguably superior configuration. The Darlington May 2010  21 Vcc B B D1 C Q1 DRIVER E (NPN) + B Vbias – A K B C Q3 POWER E (NPN) + C boost C Q1 DRIVER (NPN) E E Q3 POWER C (PNP) Vbias – IN OUT IN + R2 R1 OUT + Vbias Vbias – Vcc Vcc/2 B – E Q2 DRIVER C (PNP) B C C Q4 POWER E (NPN) Fig.3: the boosted “quasi-complementary” arrange­ment uses a “boost” capacitor to generate a voltage above VCC. This is used to drive the upper half of the output stage and allows the output to swing all the way up to the positive rail, minus the collector-emitter drop of Q3. Charge pump The circuit works by using the output of the amplifier as a charge pump. When the output swings low, the boost capacitor (Cboost) is charged up to nearly the full VCC voltage via diode D1 – let’s say to 10V. Then when the output swings high again, D1 pre22  Silicon Chip Q2 DRIVER (PNP) B BOOSTED 12V OUTPUT STAGE pairs have been replaced by compound pairs, also known as “Sziklai” pairs. Compound pairs only have a single base-emitter drop (in the drivers), so this improves the output swing to more like 10.6V peak-to-peak. Some integrated amplifiers use both these concepts. By using a Darlington upper stage and a compound lower stage, both of the high current output devices are NPN transistors. Silicon NPN transistors are traditionally better than their PNP equivalents, although this is less true now than it once was. Fig.3 illustrates this arrangement, which is known as a “quasi-complementary” output stage. It also adds a “boost” capacitor to generate a voltage above VCC, which is used to drive the upper half of the output stage. This allows the output to swing all the way up to the positive rail, minus the collector-emitter drop of Q3, which depends on the transistor size and output current. E B C Q4 POWER E (NPN) TDA7377 OUTPUT STAGE Fig.4: the output stage configuration of the TDA7377. It’s similar to the compound pair arrangement of Fig.2 but includes local gain. Because the emitters of the driver transistors are no longer tied to the output, their base-emitter voltage no longer affects the output swing. vents the capacitor from immediately discharging. Because the voltage across the capacitor stays the same, when the output swings up, Q1’s collector does too. It goes well above VCC if the output swing is large enough – in this example, nearly 22V. This higher voltage means that both Q1 and Q3 can be turned fully on, even when the output is near VCC. During the time when the output is above about 9.5V, the boost capacitor discharges through Q1 and then Q3’s base. It must be large enough so that at 20Hz it won’t discharge below 1.4V before the output swings back below 9.5V and it is recharged. This design has an output swing of 11.3V – just one diode drop away from being rail-to-rail. It’s possible to add a second boost capacitor for the negative rail but there are other techniques which provide a full rail-to-rail swing with a single boost capacitor. They usually involve making the lower output pair into an NPN Darlington and adding a more complex driving arrangement. How the TDA7377 does it Fig.4 is derived from the diagram in the ST Microelectronics data sheet and shows the output architecture used. It’s basically identical to Fig.2 (the compound pair stage) except that it also includes local gain. The main advantage is that because the emitters of the driver transistors are no longer tied to the output, their base-emitter voltages no longer affect the output swing. Consider the case where the gain set by the resistors is 10 (as in the IC) and the output is at +11V. The junction of R1 and R2 will be at 6 + (11-6)/10 or 6.5V. Thus, it’s only necessary to drive the base of Q1 up to 6.5V + 0.7V or around 7.2V in order to turn on Q1 and thus also turn on Q3. So with this arrangement there is no problem turning on Q3 until the point where the output rises to VCC. Now we can take account of the output transistor saturation and calculate just how large the output swing will be. All the previously described output stages will suffer from transistor saturation, as this depends almost entirely on the output transistors themselves. According to the data sheet, the equivalent resistance in the collectoremitter junctions of Q3 and Q4 is 0.3Ω. We can calculate that with a 4Ω resistive load and a 14.4V supply, there will be a maximum of 14.4 / 2 / 4 = 1.8A flowing through the power transistor. This will result in a collector-emitter siliconchip.com.au drop of 0.3 x 1.8 = 0.54V, meaning that the output swing under such conditions will be 13.32V peak-to-peak – not bad at all. Amplifier stability Another area where the TDA7377 has improved on previous designs is with its stability. Virtually all amplifiers with feedback systems – and this includes op amps – can suffer from instability. This is because there is always a signal delay within the amplifier. A change in the input signal does not immediately result in a change in the output. The signal is delayed by various capacitance effects inside the amplifier, mainly within its transistors. This delay, in combination with the negative feedback used to set the gain and eliminate distortion, can result in oscillation. The amplifier behaves a bit like a fish-tailing vehicle – each corrective input has a delayed effect and leads to wild over-correction. As a result, the corrections need to be damped in order to prevent this problem. In an op amp, this is usually done with an internal compensation ca- pacitor, although some (such as the NE5534) require external compensation. If an IC lacks compensation pins, a small capacitor between the inverting input and output, or between the two inputs, can do the job. However they are attached, these capacitors are configured to reduce the gain at high frequencies, where the signal delay is large compared to the waveform period. As long as the gain is below unity before the phase shift exceeds 180°, the amplifier is usually stable. The difference between the phase shift at unity gain and 180° is known as the “phase margin” and indicates how much extra phase shift can be added before oscillation will occur. For power amplifiers, stability is achieved differently. A Zobel network (also known as a “Boucherot cell”) is typically added to the output. This consists of a resistor and capacitor in series connected between the output and ground. Sometimes an RLC filter is also added, to isolate the amplifier from the capacitance of the circuitry it is driving. The Zobel network has the effect of being a frequency-dependent load. At low frequencies, the capacitor’s impedance is high, so it has no effect. However, as frequency climbs, the impedance drops to a value limited by the resistor and the loading starts to become significant. As a result, the output stage needs more current to create the same magnitude of voltage swing, reducing the gain. Thus, high-frequency oscillations are damped. We’ve already seen how the TDA­ 7377 avoids the need for an external boost capacitor or for gain-setting resistors. In addition, the boffins at ST Microelectronics have found a way to avoid the requirement for a Zobel network. How did they achieve unconditional stability? According to the data sheet, it is partially due to the way the gain is incorporated in the output stage, and partly by way of careful control over the HFE (ie, current gain) of the output transistors. They have adjusted this gain (by changing the transistor geometry) so that it is high enough to provide sufficient open loop gain for decent sound quality but low enough that runs out of steam at high frequenSC cies before oscillation begins. Radio, Television & Hobbies: the COMPLETE archive on DVD YES! NA MORE THA URY ENT QUARTER C NICS O R T C E OF EL Y! R O T IS H This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence!) right through to the final edition of R, TV & H in March 1965, before it disappeared forever with the change of name to EA. For the first time ever, complete and in one handy DVD, every article and every issue is covered. 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