LAB 3: Push-Pull Amplifier Record waveforms from the oscilloscope to a USB stick and paste these in your report. NO WAVEFORM = NO SCORE. 3.3.1 Use both lab oscilloscopes and

LTSPICE. Paste both measured and simulated transient output graphs and explain why the output does not follow the sine-wave input faithfully: [7 marks] 3.3.2 You must use lab instruments (no simulation). Paste and compare the input and output waveforms here: Has the crossover distortion gone?: Explain how the new circuit mitigates crossover distortion: Measure the actual DC voltage across both of the two diodes and note it here: [12 marks, 3ea] 3.3.3 You must use lab instruments (no simulation). Bottom bias potentiometer value here: CE Amplifier calculated gain here: Value of Rc here: Include your final circuit photo here: Initial Measured push-pull peak output ac voltage here: Tweaked value of Rc for max, barely-clipped, output here: Maximum peak output voltage before clipping is seen here: 3.3.4 [21 marks, 3ea] Demonstrate your circuit to the Lecturer in the lab. Concluding Discussion: Explain what you have seen and why you think it was so. Use theory do not simply state the obvious. NO THEORY = NO SCORE. Read the notes... [10 marks] [Max Total Score = 50 marks]/n Lab 3: BJT Push-Pull Power Stages Aim: To investigate the design and characteristics of a BJT push-push power stage. 1 Introduction Push-pull power stages are essentially bipolar common collector stages principally designed to deliver current into loads. They are typically cascaded with common-emitter amplifiers to provide a combination of voltage and current gain in one complete design. In this experiment, the basic NPN-PNP circuit is constructed in a plain Class B configuration and its behaviour is explored. An AC signal is applied to experimentally determine the gain of the circuit and investigate crossover distortion. Lastly, diodes are inserted between the bases to mitigate the crossover distortion to form a Class AB design. 2 Background Figure 1 below shows a basic push-pull stage constructed using a BC337 (NPN) transistor and a BC327 (PNP) "complementary" transistor. The bases are connected together and resistor R1 represents a load connected to the emitters of both transistors which are both effectively emitter follower circuits. Take care with the layout and ensure the emitters are connected together (look for the arrows on the transistor legs). The NPN transistor conducts during positive voltage on its base and the PNP transistor conducts when there is negative voltage on its base. This circuit, however, exhibits serious "crossover" distortion. 2 Vpk at 1 kHz ¡V3 SINE(0 2 1k 0000) g vcc Q1 BC337-25 VEE BC327-25 Q2 R1 1k Figure 1: BJT Push-Pull (Class B) Stage -VCC Holo V1 + V2 F. 15 VEE- 15 3. Procedure 3.1 Push-Pull Stage with Crossover Distortion Construct the circuit in Figure 1 on breadboard and try to scope-capture clearly the crossover distortion at the output of the amplifier across R₁. Then simulate it exactly as shown in LTSpice and compare the transient output graphs. Do as requested in Results section 3.1. 3.2 Push-Pull Stage without Crossover Distortion Now build the circuit in Figure 2 below. Two forward biased silicon signal diodes are used to force the bases of the NPN and PNP transistors apart in voltage by approximately 1.2 V. Address the questions in Results 3.2. You must use lab instruments for this (no simulation). 1 Vpk at 1 kHz V3 SINE(0 1 1k 0000) 1N4148 /VCC VEE R2 15k Q1 D1 1N4148 D2 WCC R5 15k BC337-25 VEE BC327-25 Q2 Figure 2: BJT Push-Pull (Class AB) Stage R1 1k ODAT + VEE- V1 15 V2 15 3.3 Add a CE pre-amplifier to boost input gain Push-pull amplifiers typically have a voltage gain of just 1x so they often have to be driven by pre- amplifiers to make them efficient. Maximum efficiency is achieved when the output signal max amplitude matches the supply rails without being clipped. You must use lab instruments for this (no simulation). Keeping 1 Vpk @ 1 kHz as an input signal, add a common-emitter amplifier with an ac gain of 12x to amplify this signal and then drive the input of the push-pull amplifier in Figure 2. Use a circuit like the one used in Lab 3 only using +15 V and −15 V supply rails, with 56 kỵ as the top bias resistor and then use a variable resistor as the bottom bias resistor and tweak until you set the CE amplifier's quiescent output bias voltage to +2.4 VDc. Then calculate Rc to get the gain requested. You may use a 100 µF capacitor between each stage to block DC. Now increase Rc until the output signal across R₁ just begins to clip and enter the relevant values in Results 3.3. 3.4 Conclusions A bipolar common-collector amplifier (also known as a “Push-Pull” amplifier) has been studied. The crossover distortion inherent in the pure Class B design has been investigated and a suitable method to mitigate its effect has been developed. The resultant circuit can be considered to be a hybrid between Class A and Class B and is hence called a Class AB amplifier. Discussion: In your own words, preferably backed by research and/or calculation, explain the effect introducing the diodes has on the circuit's overall efficiency. Explain also why the tweaked value of Rc is greater than expected. 123 1: Collector 2: Base 3: Emitter BC337 Pinout ^ 2 BC327 Pinout DATA 1: Collector 2: Base 3: Emitter