Design a low power amplifier for headphones- 6/9

Continuing with the designing of application specific power amplifiers, in this tutorial a low power amplifier will be designed for regular mobile headphones. In the previous tutorial, a car audio amplifier using TDA2003 IC was designed.

The mobile headphones are a type of miniature speakers designed especially for use with smart phones. These headphones do not have louder sound output. However, their sound quality can be improved by using an amplifier circuit with them. For headphones, a low power amplifier need to be designed. The amplifier need to be designed in a way that its audio output remains noise free and controllable particularly at peaks (high amplitudes or loud sounds). It needs to be taken care that even after amplification, the volume should remain at comfortable level as louder sound output can be unpleasant with the headphones and may have more noise as well.

When an amplifier is designed for any typical speaker, the amplifier circuit has to deal with a narrow range of load impedance typically 4 ohms to 8 ohms. So, it is easy design amplifier for regular speakers. However, when an amplifier circuit has to be designed for headphones, there comes a major problem due to difference in standards followed by different manufacturers. The headphones from different manufacturers have different impedance. Generally, impedance of a headphone can range anywhere from 16 ohms to 600 ohms. This creates a serious compatibility issue. Also headphones having lower impedance of 32 ohms or less can operate with most of the devices but headphones having higher impedance may not work with all the devices.

In this tutorial, a general purpose low power audio amplifier for a headphone of 30 ohms impedance will be designed. The amplifier circuit is designed using LM741 OPAM IC. LM741 is a general purpose operational amplifier IC. The amplifier circuit will be operated by a 9V battery. The power output of this amplifier circuit will be 1 Milli Watt. In fact, LM741 can have a supply voltage up to 22 V and input signal amplitudes (input voltage) up to 15 V.

In the introductory article of this series, various design parameters of the audio amplifier circuits were discussed like Gain, Volume, Skew Rate, Linearity, Bandwidth, Clipping effect, Stability, Efficiency, SNR, Output power, THD and loop grounding. This amplifier circuit will be designed considering the following design parameters –

Gain (Voltage) – 20 dB

Bandwidth – 20 Hz to 20 KHz

Output Power – 1 mW

The amplifier will be designed to supply audio to a headphone having an impedance of 30 ohms. The circuit will have the following additional features –

– No Clipping Effect

– Volume Control

The designing of the circuit will be followed by testing of the circuit for the verification of the intended design factors.

Components Required –

Fig. 1: List of components required for 741 OPAM IC based Low Power Headphone Power Amplifier

Block Diagram –

Fig. 2: Block Diagram of 741 OPAM IC based Low Power Headphone Power Amplifier

Circuit Connections –

The amplifier circuit is built by assembling the following components together –

1) DC Source – There are two sources of power required in this circuit. A power supply of 5V is required for biasing of microphones while a power supply of 9V is required for the OPAM circuit. The 9V DC source is needed to provide the bias voltage to the amplifier. For making the 5V supply, two 9V batteries are connected in parallel.

2) Audio Source – An electret MIC has been taken as the input source. The electret MIC requires a biasing voltage between 1 to 5V to power the inbuilt FET buffer which is present in the MIC.

Fig. 3: Typical Image of Electret Microphone

Generally, this microphone is powered by 1 V to 5 V DC through a resistor having value from 1K ohms to 10K ohms. The biasing circuit for this microphone has been shown below –

Fig. 4: Circuit Diagram of Electret Microphone Biasing

3) LM741 Operational Amplifier IC – The LM741 is a general purpose OPAM (operational amplifier) with a low input impedance (mega ohm) as compared to FET OPAM. The FET OPAM has a high input impedance in Giga ohms. The output impedance of 741 should be ideally zero but it is usually about 75 ohms. The maximum supply current of 741 IC is about 2.8 mA with a supply voltage up to +/- 18V. The IC has the following pin configuration –

Fig. 5: Table listing pin configuration of LM741 Operational Amplifier IC

The IC has the following pin diagram and the functional diagram –

Fig. 6: Pin Diagram of LM741 Operational Amplifier IC

The IC comes with overload protection on both input and output plus has no latch up when common-mode range is exceeded. The IC can be provided a supply voltage positive or negative up to 22 volts and input signal voltage (amplitude) can be up to 15 V. Under normal conditions, the IC must be provided a voltage of 10 V either positive or negative. The IC has the following internal circuit –

Fig. 7: Internal Circuit Diagram of LM741 Operational Amplifier IC

The LM741 can be configured as both open loop as well as closed loop amplifier. It can be used as either inverting amplifier or non-inverting amplifier. In this circuit, the LM741 IC has been used as inverting amplifier as the input signal from the microphone is applied at the inverting input pin (pin 2) of the IC. The inverting amplifier has negative feedback which makes them better than non- inverting amplifiers. The inverting amplifier changes the phase of output (signal amplitude) by 180 degrees with the input (signal amplitude). However, this phase inversion does not affect the audio signal as human ears are response only to the intensity of the sound. The intensity is the energy flowing through an area in a given time, expressed in joule/s/m2. The energy of the wave is proportional to the square of its amplitude. So for a unit area, the intensity is also proportional to the square of the amplitude.

I A2

Changing the sign of wave has no effect on I.

Fig. 8: Typical Image of LM741 Operational Amplifier IC

The audio input from the microphone is passed to the inverting input pin of the LM741 IC through a variable resistor (Shown as RV1 in the circuit diagram). This variable resistor helps in adjusting the amplitude (voltage level) of the input audio signal, so it also adjusts the volume of the output signal as well. The volume or amplitude of the output signal remains linearly proportional to the input signal amplitude by the voltage gain factor.

The supply voltage to the IC is provided at pins 4 and 7 through filter capacitors. At pin 4, two capacitors (Shown as C4 and C5 in the circuit diagram) of 100 uF and 0.1 uF are connected while at pin 7, two capacitors (Shown as C2 and C3 in the circuit diagram) of 100 uF and 0.1 uF are connected. The high value capacitors like C2 at pin 7 and C4 at pin 4 helps in filtering high frequency from the supply voltage and low value capacitors like C5 at pin 4 and C3 at pin 7 helps in filtering low frequency from the supply voltage. The negative feedback is provided to the amplifier through a resistor (Shown as R3 in the circuit diagram) of 100 K ohms. At the output pin (pin 6) of the amplifier, a filtering capacitor (Shown as C6 in the circuit diagram) is connected to block any DC components from the amplifier to the headphone as DC components (due to clipping effect) can damage the headphone or add noise and distortion in the audio output.

4) Headphone – The amplified audio output is taken from the pin 6 of the IC. A headphone having an impedance of 30 ohms is used as the output load. The headphone is connected to the circuit by using a female audio earphone jack. One of the wire from the female jack is connected to the output pin of the amplifier IC while the other wire from the female jack is connected to the common ground. The output impedance of the amplifier should not be too high for a headphone having lower impedance. So, the LM741 has usually output impedance of 75 ohms which is suitable for a headphone having an impedance of 30 ohms.

Fig. 9: Typical Image of Mobile Headphone with female audio jack

The following precautions must be taken care while assembling this circuit –

1. Always place the components as close as possible to reduce the noise in the circuit

2. Follow star topology when grounding, this will keep the noise low.

3. Use the capacitor of high voltage rating than the input signal.

4. Always use the filtering capacitor at the input terminal of power supply to avoid the unwanted ripples.

5. Use the speaker of equivalent or high power rating as amplifier output power.

6. Always use a series capacitor at the output of the amplifier to block any DC component.

7. Always calculate the maximum power rating of the amplifier before connecting it to the speaker. The practical value may differ from theoretical one.

8. Avoid clipping of the output signal as it may damage the speaker.

Fig. 10: Prototype of 741 OPAM IC based Low Power Headphone Power Amplifier

How the circuit works –

The LM741 operational amplifier has been configured as inverting amplifier in closed loop configuration. The input signal from the electret MIC has an amplitude in range from 1 mV to 10 mV when the user speaks in normal voice. In the case of screaming the input level may increase up to 50 mV. So the average value of input signal can be considered 20 mV. The maximum voltage that should be obtained at the output is 200 mV. So, the desired voltage gain can be calculated as follow –

Gain = Vout/ Vin

since, Vout = 200 mV and Vin = 20 mV

Gain = (0.2/0.02)

Desired Gain = 10

So, the desired gain is 10 or 20 dB. In the circuit, the voltage gain can be set through the resistor network R2 and R3 where gain can be calculated as follow –

Gain = -(R3/R2)

If resistor R2 is assumed to be 10K ohms and desired gain be 10 or 20 dB, then the value of resistor R3 is obtained as follow –

R3 = Gain * R2

R3 = 100K ohms

The negative sign signifies the inversion at the output. So, a resistor of 10K ohms is used as R2 and a feedback resistor R3 of 100K ohms is used in the circuit. The amplifier gives the output signal which is 10 times the input signal.

Testing the circuit –

For the testing of the amplifier circuit, the function generator is used as the input source. The function generator is used to generate a sine wave of constant amplitude and frequency. Any audio signal is also basically a sine wave so a function generator can be used instead of using a microphone or actual audio source. So, the function generator can be used as input source for testing the audio amplifier circuit. During testing, at the output also, a headphone is not used as a load as the headphone speaker is resistive as well as inductive. At different frequencies, its inductance changes which in turn changes the impedance (R and L combination) of the speaker. So, the use of a headphone as load at the output of the amplifier for deriving its specifications may give false or non-standard results. In place of headphone, a dummy load which is purely resistive is used. As resistance does not change with frequency so it can be considered a reliable load independent of the frequency of the input audio signal.

For testing of the amplifier circuit, first the input voltage is set between the applicable range between 10 mV and 50 mV. The frequency of the input signal is set to 1 KHz. Then, the output waveform is observed at CRO and the input signal is increased until the output waveform starts clipping.

With dummy load of 20 ohms impedance, the following observations were noted –

Fig. 11: Table listing output characterstics of 741 OPAM IC based Low Power Headphone Power Amplifier

It was observed that the output voltage started clipping at 200 mV. So, the output power of this amplifier can be calculated as follow –

Output power, Po = V2(p-p)/2R

Po = (0.2*0.2)/(2*20)

Po = 1 mW

So, this amplifier has a maximum power output of 1 mW. On observing the input and output waveforms of the amplifier on a Digital Storage Oscilloscope DSO, the following waveforms were observed.

Fig. 12: Graph of Input and Output Waveforms of Headphone Audio Amplifier observed on Digital Storage Oscilloscope

In the above figure, the yellow waveform is the input waveform to the amplifier circuit while the red waveform is the output waveform from the amplifier. It can be observed that the input signal is almost amplified 10 times at the output of the amplifier. Also, there is no clipping effect in the output waveform.

This headphone amplifier is simple and cheap to design. It has a small circuit and can be assembled in a compact space. This is an exciting Do-It-Yourself circuit to try and can be used with a regular headphone of any smart phone.

In the next tutorial, a stereo amplifier will be designed. Until now, amplifier circuits were designed for single audio channel. In a stereo audio system, there are many audio channels output through different speakers forming a surround system. For such audio systems, stereo amplifier which can amplify multiple audio channels is required.