# Sine Wave Amplification

Discussion in 'The Projects Forum' started by RICTom, Apr 11, 2009.

1. ### RICTom Thread Starter New Member

Apr 10, 2009
1
0
Hello,

I ultimately would like to take a 1Vp-p sine wave and amplify the voltage and current so that there is 4-5Vp-p and 500mA at the output to drive a 7ohm device. I have a +5V power supply from a USB port that can source 500mA.

I have implemented a 741 with a 2.5V DC bias to essentially create a 50% duty cycle PWM to drive a BJT to supply the current amplification, but I am curious if there is any other way to amplify the voltage and current with a 1Vp-p sine wave and a single +5V power supply???? Any suggestions and/or tips are welcomed.

Thank you.

2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,791
5v is not sufficient differential for a 741 opamp's supply voltages. Also, you are probably using the 741 in open-loop mode (no feedback; as a comparator) which causes the output circuitry to be in constant saturation, leading to high power dissipation and heating of the IC.

You won't be able to output 500mA current from a USB port unless you're using MOSFETs, with a 100% efficient circuit, which is just about impossible. You might get close, but it'll never be 100% There will always be some power used in the driver circuit.

Since your load is 7 Ohms and the maximum available current from a USB port is 500mA, the most voltage you'll see across your load is 3.5v (Ohm's Law; E=IR, or Voltage = Current x Resistance). If you don't provide at least an additional 3 Ohms in series to limit the current somewhere, you risk causing damage to your USB port.

You might consider using one channel of an L2722 low-dropout dual power opamp. These are capable of 1A continuous, 1.5A peak current, and have a dropout of around 1v at 500mA current. You would need at least a 1 Ohm 2W resistor in series with your output. You would not need the transistor anymore, as the L2722 can drive the load directly.