Push-Pull Power Amplifier Design with Op Amp, Sziklai Darlington Transistors

How to Design a Push-Pull Power Amplifier Circuit with Op Amp and current boosting Sziklai BJT Power Transistor is discussed in this electronic power amplifier circuit design example including the analysis of the circuit operation and proper choices of components in this design. For more amplifier example see • Electrical Engineering... . This Push Pull Amplifier is designed with one Op Amp and a complementary pair of PNP-NPN Sziklai and NPN-PNP Sziklai BJT transistor pair (a variant of the Darlington transistor pair). There are also small current-limiting PNP and NPN Bipolar Junction Transistors in this circuit that combined with the current limiting resistors protects the output sensitive and expensive power transistors from burning by limiting the output load current to a predetermined value. The Operational Amplifier in this design is wired in the form of a non-inverting amplifier to provide the desired voltage gain and also the needed negative feedback in this class AB power amplifier circuit. A combination of op amp virtual short, Kirchhoff Circuit Laws (KCL and KVL) and transistor current gain is applied to analyze this amplifier and find the proper choices of circuit components.

[@STEMprof]

Thanks for watching. For more Op Amp Circuit examples pls see:

[@nareshsaket5401]

Thanks sir

[@debrainwasher]

A nice Sziklai Darlington circuit, but I have some stability doubts. The first one is, Op Amps normally need capacitors at the supply rails to avoid random phase shifting of the internal components (e.g. current source of the differential amplifier). A phase-shift close to π turns every amplifier with feedback into an oscillator, resulting in magic smoke from the power transistors. Already a typically used 220nF capacitor with R3=70Ω results in maximum a frequency of 10.3kHz (-3dB-point) - instead of 20kHz due to τ=RC and fo=1/(2πτ). The second issue is the stability of the bias current to suppress crossover-distortions. Since BE-voltage of a BJT drops about -20mV/K according to the Shockley-equation, I can't recognize a stable and temperature compensated bias. In my power-amplifier designs I regularly use a either two diodes and a variable resistor, or a BJT with a variable resistor, thermally coupled to the power transistors. This ensures a stable bias-current over the whole intended operation temperature range.

[@wliterow]

very nice work - congratulations on an exceptional presentation - perhaps can you present it in L spice

[@rolfts5762]

Could it be better to re-considered this circuit?: ..Perhaps better to have the OpAmp feed towards the upper&lower sziklay-pairs instead(also make some biasing for the first transistors of those), ..And also perhaps removed both the R3's (that potentially can interrupt stable Vcc/Vdd for the OpAmp).. /The upper&lower 'clamping-transistors' seems like a fine concept (not referencing towards them, they seem Ok).

[@dineshkumani715]

सर जी, बहुत ही बढ़िया,,पर हिंदी भाषा में होगा तो ज्यादा लोगों को आप की जानकारी का लाभ मिलेगा

[@sc0or]

For me to have a good result, we need another really good sounding input opamp and a common negative feedback loop. Because all these "special" (high voltage) opamps are total cr.. considering how do they sound. Another thing is a setting of a bias on the output transistors. I think the opamp output resistor can do this. It must be small. The third thing is a stability. Shiklays are not good in this. Must be simulated, prototyped and tested.

[@struberg]

I have a hard time understanding where the 'control' of the circuit comes from.

[@Chupacabras222]

Very interesting schematic. I'm missing one thing, how to determine value of output resistor for that opamp? I would proceed this way: define max allowable current of opamp (say 30mA in this case), if output is shorted then max voltage would be 35V. So value would be 1.2 kΩ. Or I have to consider full span ±35V (in case there is some capacitance at the output and charged to -35V), so the value would be 2.4 kΩ. Or there is something else to consider?

[@szekerespista3758]

Dear Mr, could you please recommend a ultra low impedance output amplifier (with 1 ohm output)?

[@AnalogDude_]

I like this circuit alot, would like to use it for a pc speaker, but i can offer max +/-15V.

[@hadighorbani355]

like