Two-Stage Audio Amplifier

Overview

Designed and built a two-stage audio amplifier. The brief required a circuit capable of driving an 8 Ω speaker to a specified output level from a single 9 V supply, with a frequency response covering the full audible range. I chose a two-stage topology: a TL071 op-amp configured as an inverting active band-pass filter in Stage 1, followed by an OPA551 unity-gain voltage follower in Stage 2 to supply the current the speaker demands without the first stage having to work against a low-impedance load.

Stage 1 achieves a gain of 10.67 dB across a passband of 6.63 Hz to 28.54 kHz, with the upper and lower -3 dB cutoff frequencies set by the RC networks around the op-amp. Vcc/2 virtual ground biasing centres the signal on 4.5 V so the single-supply circuit swings symmetrically. A 2200 µF output coupling capacitor blocks the DC bias from the speaker. Stage 2 uses the OPA551 because it can source up to 200 mA continuously, well above what the TL071 can supply and enough to deliver 281 mW into 8 Ω. Reverse polarity protection is handled by two 1N4007 diodes in series with the supply rails, an LED power indicator and a power switch complete the auxiliary circuitry.

I worked through three build stages before committing to PCB. The circuit was simulated in Proteus with ideal and then real component models, then breadboarded on a dual-supply bench supply to verify gain and bandwidth independently of the biasing network, then rebuilt on a single 9 V supply to confirm the Vcc/2 biasing held stable under load. Only after both breadboard versions matched simulation did I move to PCB layout. The final board is 65 mm x 40 mm, laid out in Proteus with a continuous ground plane, 45-degree mitred track corners and a DRC-clean design. The assembled and powered PCB measured a 2.980 Vpp output against a 3 Vpp target, a 0.67% error.

Key highlights

• ·Two-stage design: TL071 active band-pass filter (Stage 1) and OPA551 unity-gain buffer (Stage 2)
• ·Gain: 10.67 dB, passband: 6.63 Hz to 28.54 kHz, output: 2.980 Vpp into 8 Ω (281 mW)
• ·Single 9 V supply with Vcc/2 virtual ground biasing and AC coupling
• ·Reverse polarity protection, LED indicator and on/off switch
• ·65 mm x 40 mm PCB laid out in Proteus with ground plane, mitred corners and DRC-clean design
• ·Validated across Proteus simulation, dual-supply breadboard, single-supply breadboard and PCB

Technologies