Question

OUESTION 1 Anna is designing an audio amplifier for her headphones. She wants to calculate the real DC gain of the amplifier before putting music through it to make sure it won't destroy her headphones. She has setup a test circuit of the non-inverting amplifier shown below. She has used a cheap poor quality op-amp where the input resistance is 1 kQ, the output resistance is 4 Q and the open loop gain is 100xvin 2Q2 ww 4 2 V V Vin >1 kQ 100Vin 10 Q Rspeaker 20 2 (a) Assuming the golden rules of an op-amp don't hold for this circuit what is the current, iim, entering the non-inverting terminal of the op-amp? (10 marks) (b) Knowing the input current, what is the output voltage, Vo, and hence calculate the actual gain of this amplifier circuit? (15 marks) (c) Anne would like to know how the amplifier acts when the headphones are connected between terminals a and b. Calculate the Thevenin equivalent source of the amplifier between terminals a and b (10 marks) Knowing the equivalent circuit of the amplifier Anne would like to choose headphones that would enable the amplifier to transfer the most power to them. Anne has the choice of using headphones that have a resistance of 2 Q, 4 Q or 8 Q. Which headphones should Anne choose and why? (d) (10 marks) (e) If Anne connected the speaker that you selected to the amplifier, what would the power being dissipated through the speaker be? (5 marks) (TOTAL: 50 marks)

Answer 1

CIRCUIT DIAGRAM 3v 100Vin Tol ttin) MAN 202 Ansa)Appling in he loop KVL вй to12 again r ling KVL 4xi-20 - oo Vin (1кяin) -4тi- 2оri (i) Solving 0.24 A q.86 xio A (Ah in 9,86 X10 А

Ang b)pplygKvL 20 3 0 K 0.29V g.4 Grain- 2.4 z 8.7 Volt VTH &c) Thuenin's quivalant sautce veltagu Ans b3nne dhing cheuiting IV SoutL found Thauenin's Ruistanu As Vin shont acsoss a b 22 Vin IKL 00Vin 102 2.0. whih s CIKA 2) ry hgh o A 3 0 2010 Sina lino A 2 0.283 *Total (urent 30 3.5n RT2

ckt Fqulvalent RTH 3-5 Rspeah AŁ-8= HIA speakur gulston u 4 (d) ahoaue Anne should at this usistanu powuud dilfvered wil b moe than 2N a, 5.38 Watt. Rspakr in hsptaku Powe dissipaad 8.7 5.3824 Watt