DNY40W Block and Schematic Diagram :
The DNY40W power amplifier block diagram is shown in Figure 4 and the Schematic diagram is shown in Figure 5. I took the liberty to expand the block as:
- Input Stage
- Differential / Constant Current Source 1
- Feedback Network
- Voltage Amplifier / Constant Current Source 2
- Output Transistor Bias Control
- Driver Transistor
- Output Transistor
- Zobel Network
- Power Supply / Grounding
Let’s see the function of each stage, refer to the block diagram in Figure 4 and then refer to the actual circuit diagram in Figure 5.
(1) Input Stage (R2)
The Input Stage abbreviated as “IS” in Figure 4 is the interface to the tone control or music player, it is called the “input” — a low level electrical “audio signal” typically 700 mVrms (0.7Vrms) to about 1.4Vrms.
The input impedance (Zin) is effectively the resistance of R2. The R2 along with C2 form the high frequency low pass filters to reject high frequency (RF), a frequencies above the audio spectrum of 20Hz to 20 KHz.
The R2-C2 filter prevent this high frequencies from entering the Differential Amplifier stage so that the effective signal to be process is only the audio frequencies.
The capacitor CA is a coupling capacitor which prevent dc voltage (block) to be transferred into the music source, or the music source transfer dc voltage into the differential amplifier stage. Its use is important to make it sure that, the dc bias of the differential amplifier will not be disturb.
2) Differential Amplifier (Q1, Q3)2) Differential Amplifier (Q1, Q3) Constant Current Source1 (Q2, D1.D2). The differential amplifier (Q1 and Q3) accept input voltage signal and output a current signal, it provide the initial amplification of the input signal (0.7Vrms to 1.4Vrms), it control the offset voltage, distortion and making the stability of the amplifier in conjunction with the negative feedback network.
The requirement of the differential amplifier (Q1 and Q3) for a good Offset Voltage is a good pair (equal performance) of PNP transistor. The offset voltage is the voltage appears at the speaker output, theoretically it is zero volt dc (0 Vdc) but in the real world it is not, it plays as high is +/- 200mVdc, which means this level of dc voltage of 0.2Vdc is positively “offset” from zero – it is not acceptable. For the sake of engineering quality, our team set the offset voltage to maximum +/-50 mVdc (+/- 0.05Vdc).
The dc voltage at the speaker should be minimum, a high dc voltage at the speaker may tend to damage the speaker in prolong use, that is why it is very im
portant to test and meet this specification, it must be observed religiously. It is the first test to do – – – to know the dc voltage appearing at the speaker before putting the Power Transistor Q10 and Q11 and adjusting its bias voltage (Q7 via VR1).
The use of Constant Current Source (CCS), I called it here as CCS number 1 or CCS1 specific for the constant current for the differential amplifier stage, theoretically the CCS1 will introduce an infinite source impedance …… please note, that, in the real world, it only approaches ideal – – – – thus, our CCS1 will only achieve a very high impedance seen by the emitter of the differential amplifier which will make it to have a high CMRR (Common Mode Rejection Ratio). Having a high CMRR means – – a better performance which has the ability to “reject” a “common” signal or any noise at the differential inputs “a” and “b”.
The CCS1 mechanism is to have a constant voltage across R4 (in Figure 4), this can be achieve with the
diode D1 and D2. This will provide a theoretical voltage across D1-D2 of 1.4Vdc (perhaps it is 1.238Vdc in the real world as measured by your digital voltmeter), which in effect will provide 0.632Vdc across R4 (instead of 0.7Vdc theoretically).
The CCS1 is “source” the current to the emitter of the differential amplifier, which simply means, the current is “going into” the emitter of the differential amplifier Q1 & Q3, an important to differentiate when you may see a CCS using a NPN transistor which will be “sinking current” instead of “sourcing”.
(3) Negative Feedback Network (R8. R6)
In conjunction with the differential amplifier is the Feedback Network, which is composed of R8 and R6 – – – a path of the output signal is being fed back into the input “b” of the differential amplifier. Basically these resistors will determine the “close loop gain”.
(4) Voltage Amplifier (Q6)
It is also commonly called VAS (Voltage Amplifier Stage). It accept current as the input from the differential amplifier output and it will output a high voltage swing at its collector which will drive the Driver Transistors Q8 and Q9.
Obviously, the gain of the differential amplifier Q1 and Q3 is not enough to drive the driver transistor that is why a high gain voltage amplifier is used (Q6). In the circuit in Figure 5, the VAS is composed of a Common Emitter, this configuration provides the greatest voltage gain (as compared to Common Base or Common Collector) with high input impedance. The requirement of high input impedance is required not to load the previous stage – – the differential amplifier output of Q1.
The use of Constant Current Source 2 (CCS 2) will improved the gain of the voltage amplifier seen as a load — the impedance of the Constant Current Source (CCS) is very high, ideally it is infinite, in our real world that is only approaching to the ideal.
In Figure 5, you will see C3 connected between the base and collector of Q6, basic textbook shows that this is called Miller Capacitance (Miller’s effect) named after John Milton Miller. This component is important for an amplifier (Inverting amplifer as in Q6 – Common Emiter) which will limit the gain of the amplifier, which mean, as you increase the input the output become distorted, this is so because of the effect of the parasitic capacitance between the input and the output, specially at high frequencies. The value of C3 may vary between electronic design and PCB design – the transistor used in Q6 and the effect of PCB trace on Q6, which means, it is dependent on the “total effect of capacitance”. In this design, a 10pF capacitor is used.
If you changed the electronic design or pcb design, the C3 capacitance “maybe” changed as well and thus the 10pF may no longer applied, maybe a different value is required.
You may see also in Figure 5 of the capacitor C4 at the output of the VAS, the function of this capacitor is to stabilized and kill the high frequency oscillation when all the power transistor is connected. Without it, the power transistors will gets hot easily and may burn it (damage it). It is placed at the VAS stage as this stage is processing the highest gain of amplification.
(5) Driver Transistor (Q8, Q9)
The Driver Transistors will “drive and provide enough current” into the Output Transistor (Q10 and Q11) so it can effectively provide the needed current to have a maximum voltage swing into the Speaker (Load).
Obviously, the VAS output current is not enough to drive the Power Transistors. . . . . I would rather say, the current output required by the VAS to drive the Output Transistors would be large, and that makes the VAS very hot and requires higher power handling capability transistor – – knowing it is already operating in Class A, that makes Q6 continuous conduction (well…. with the excemption if you will use a real world darlington Power Output Transistors – as you may know darlington comes in package with each driver transistor).
With the rule of thumb on reliability, the driver transistor Q8 and Q9 were used and makes the VAS ouput current lesser (means, less hot generation at Q6 and use of lower power transistor) making the design a reliable circuit and better performance.
You may see again the Miller Capacitance at the B-C junctions of the driver transistors, which is the same used as explained earlier.
(6) Output Transistor (Q10, Q11)
In conjunction with the Driver Transistor (Q8 and Q9), the Output Transistor (Q10 and Q11) provides the current demand into the speaker (load).
You may noticed that it looks like a tag-team of transistors populary called “darlington”, most engineers called this configuration as Emiter Follower, as they are not real darlington Transistors, as opposed to the “real darlington transistors” which packed in one package.
The use of Emiter Follower configuration was voted for the sake of cost, as real darlington transistor are relatively expensive compared to the configuration of Emiter follower.
The Output Transistors must be place into a suitable heatsink to effectively dissipate the heat generated during the amplification. It is expected that, 25% to 30% of power were converted into heat during the amplification process at the Power Transistors.
If you do not put the power transistors into a suitable heatsink, it will get hot, and very hot and may end up in what is called thermal run-away that damage the output transistors and will burn your speaker – a dangerous scenatio that you may not like it.
(7) Bias Control for the Output Transistor (Q7)
The use of this transistor is to set the output current of the Power Tansistors Q10 and Q11 by controlling the input current into the driver transistors (Q8 and Q9). The Q7 is called Vbe multiplier – we loved to called it Bias Control.
By setting the input current to the driver transistors, you can make the Output Transistors to operate in class A , class B or in between class A and class B, known as class AB operation.
In class A, the output transistor keeps on conducting (always have current at the collector like 70mAmp to 100mAmp) with or without input signal at the differential amplifier at point “a”.
In class B, the output transistor will not always conducting or not always have collector current. If there is no input signal at point “a” at the differential amplifier stage, the collector current is practically zero, but the collector current is maximum when a maximum input signal at the differential amplifier point “a”.
In the class AB operation, the output transistor have an idle Collector Current, like 10mAmp or 25mAmp or even higher – just remember that the higher the idle current – the more hot the output transistors and requires bigger heatsink. Let’s understand it.
Setting an idle current is desirable in order to avoid the crossover distortion (class AB). This is a distortion or “dead signal” when the audio electrical signal – – positive signal and negative changes polarity.
Consider a pure sinewave of 1KHz frequency as shown below, during the positive cycle, the Transistor Q8 and Q10 conducts,
During the negative cycle, the Transistor Q9 and Q11 conducts,
Combining the 2 signals, you may see a dead signal while changes polarity from positive to negative.
By adjusting the Bias Control (Q7) thru VR1, the input current to the output transistors varies and the dead signal will be eliminated (making the cross over distortion minimized or completely eliminated).
The effect will be a cleaner music as heard on the speakers specially for the high frequencies in the range of 4KHz to 16 KHz (20 KHz if ever you can hear it), especially at low volume (low input signal at the differential amplifier point “a”).
As a real bias “Bias Control” in any situation, (a) at normal temperature of Q10 and Q11 or (b) at hot temperature of Q10 and Q11, the Q7 will act as servo mechanism to countermeasures the temperature changes of the output transistor, Q10 and Q11, as :
(1) Power Transistor Hot – decrease the Bias
(2) Power Transistor Warm – increase the Bias
in this way, a thermal run-away can be avoided by decreasing the bias when the transistor gets hot. Without a servo mechanism, when the output transistors get hot, the bias keep the same level then the output transistors keeps on getting hot and becoming hotter which leads to thermal run away.
To effectively let Q7 do its job and perform real action, it must be installed into the heatsink where the output transistors are installed.
In most design, the driver transistors (Q8 and Q9), the output transistors (Q10 and Q11) and the bias control transistor (Q7) are placed in one heatsink.
Although, Q7 do not process the audio electrical signal as the differential and VAS stage, it is there to activate (set proper bias) the driver and output transistors into conduction to deliver the “power” needed at the load. Similarly, the CCS 1 and 2 do not also process the audio electrical signal, they are there for the proper operation of the circuit which process the audio electrical signal.
(8) Zobel Network (R18, C7)
This RC network R18 and C7 will make the system stable with the speaker or when the speaker is removed.
The speaker (or speaker system) is composed of an inductor, its impedance become higher when the frequencies goes higher, this is also the same when you remove the speaker, the speaker (load) is now a very high impedance which tend to destabilized the amplifier — it “might oscillate”.
To prevent this from happening, the Zobel Network is in place to countermeasures the scenario by introducing low reactance of the capacitor C7 which in effect shunt the Resistor R18 thereby lowering the total load impedance. With this effect, the whole system is much stable in any case.
(9) Speaker Wire Capacitance (9) Speaker Wire Capacitance Killer (SWCK) (L1, R19) The SWCK (I love to call it this way–”swack”) is the “capacitance killer” to stabilized your power amplifier when you use long speaker wire (maybe 10 meters or more) that it creates capacitance due to the proximity of the wires in pair.
The capacitance is then a part of the speaker (load) which is worst at high and low frequencies. To kill the capacitance caused by long wires, the inductor L1 is introduced, as you may know, the inductor is the opposite of capacitor. But there is problem with an inductor – the Q, it tends to be a series L-C which when tuned become an oscillator and worst is the impedance is approaches zero.
To make it operational without issue, the R19 iis introducedto kill the high Q, when it is shunt into the inductor.
(10) Speaker (Load)
We know already from Figure 2 that the speaker will convert the electrical audio signal into sound waves which our ear can understand. The speaker is the sum total of all the characteristics of the amplified audio signal, it is where we listen to the fidelity of the amplified music.
The speaker should be observed in terms of “proper loading”, our amplifier was design with 8 ohms impedance so the speaker to be used must be 8 Ohms.
(11) Power Supply and Grounding Systems
The power Supply is one of the main important ingredients of any power amplifier which is the “source of power”.
If the power supply is not well designed – not well filtered, have rippled / noise, the power amplifier sound quality will severely affected specially the bass (low frequencies below 200 Hz). If there is no enough high frequency filter it may result to several issues like damp oscillation or even amplifying RF.
Along with proper grounding system, it makes the power amplifier stable, humless, noiseless and motorboatingless amplifier. A wrong implementation of grounding can heard its affect on the speaker.
To improved the noise, the power supply ground (PSU_GND) and the analog ground (A_GND) were separated and meets at one point by R1. This technique is pretty important specially when integrating the power amplifier into different boards/module like tone control, mp3 player, bluetooth module, etc.