Spis treści:
Operational amplifiers are some of the most important components in analog electronics.
It is with their help that we can perform various mathematical operations on electrical signals.
In this article, we will look at how the TL072 operational amplifier from Texas Instruments, can come in handy for us to realize electroacoustic circuits.
Key parameters of the TL072 operational amplifier
- Number of channels: 2
- Voltage rise time: 13V/µs
- Input unbalance voltage: 1mV
- Temperature drift of non-equilibrium voltage: 2 µV/°C
- Current consumption: 940 µA/channel
- Noise level: Vn = 18 nV/√ Hz at f = 1kHz
- Total harmonic distortion THD: 0.003%
- Supply voltage range: ±2.25V – ±20V or 4.5V to 40V
TL072 - headphone amplifier application
Headphones have been one of our everyday items for many years, just like many other consumer electronics devices that pass through our hands on a daily basis.
What’s more, probably no one likes poor sound quality, where sometimes it could seem that the signal-to-noise ratio is close to unity or even less than unity – for this reason, in order to get the best possible sound quality, it is worth building your own headphone amplifier.
Such a device can be built using readily available low-cost electronic components, such as the J-FET TL072 dual operational amplifier manufactured by Texas Instruments.
TL072 amplifier features a low input unbalance voltage (typically 1mV), a high voltage rise rate (13V/µs) and a low co-signal attenuation factor with respect to the positive supply voltage rail.
For these reasons, the TL072 is a great choice for applications where the key low cost of implementation and high efficiency of operation.
Prezentowany tutaj obwód wzmacniacza słuchawkowego Hi-Fi, wykorzystuje niskoszumowy wzmacniacz operacyjny TL072 do selektywnego wzmacniania sygnały z kanału lewego i prawego – ponieważ oba kanały słuchawek są takie same, wyjaśnimy jak działa po prostu jeden z nich.
At the input J1, an input signal is applied, which passes through capacitor C1 designed to eliminate the DC component from the input signal.
The volume of the input signal is adjusted by potentiometer VR1.
This results in an output impedance of 47k, which is equal to the resistance of potentiometer VR1.
In this circuit, both channels of the TL072 operate in a non-inverting amplifier configuration.
The output of the left channel is at pin 1 of the TL072 amplifier, and the polarizing current is applied to the bases of transistors Q1 and Q2, and resistors R3 and R4 configure these transistors to operate in class A. The most important elements of the topology working in class A, are two other resistors viz.
R5 and R6.
These are needed to improve the stability of the output, setting the level of the feedback signal there.
The inverting input of the TL072 operational amplifier receives the feedback signal when resistor R2 is turned on.
The values of resistors R1 and R2 determine the gain of the operational amplifier.
TL072 in the role of microphone preamplifier
Based on the TL072 IC, we can also build a microphone preamplifier.
The principle of this circuit is as follows – resistor R1 supplies power to the electret microphone forming part of the FET amplifier.
It is recommended here to use a resistor of 2k to 10k, but the higher the value of this resistance, the better the channel separation.
Another microphone can draw polarizing current from the same supply voltage rail.
It turns out that higher values of this output resistance also reduce distortion, and the best polarization circuits are obtained by cutting a path located on the underside of the electret microphone housing to allow the connection of an additional drain and source resistor.
A 2.2uF capacitor blocks the constant polarizing voltage from the input, and combined with a 27k resistor, creates a high-pass filter.
The output impedance of the circuit is set by two 27k resistors and a 10k resistor.
The positive supply voltage rail is connected to ground through a 47uF capacitor.
The 27k resistors form a voltage divider that applies half the supply voltage to the non-inverting input of the operational amplifier.
Relevant here, therefore, are two 27k resistors connected in parallel, giving a resultant resistance of 13.5k, in parallel with a 10k resistor, making the input impedance approx.
6k.
If we provide the correct dual supply voltage, we will not need the top 27k resistor, since the input no longer needs to be polarized with respect to ground.
The feedback loop here has two resistors 27k and 1.5k connecting the inverting input to ground.
When both resistors are in circuit, the gain is just under 2.
The 27k resistor can be shorted out with a switch – then only the 1.5k resistor determines the gain by setting it to 23.
A 10uF capacitor in the lower half of the feedback loop reduces the DC gain to approx.
unity.
If any DC input offset is amplified, it would cause a larger offset at the output, pushing the output level toward one of the rails and reducing the supply.
At a gain of 23, with the expected input levels, this probably doesn’t matter.
An optional 2pF capacitor, relative to a 33k resistor, sets the high-frequency drop.
The cutoff frequency is in the hundreds of kHz.
It needs to go further than 20 kHz to keep the phase shift small at audio frequencies, and also because the output power starts to drop long before the signal is cut off.
Operational amplifiers can’t maintain enough gain at these frequencies anyway, and their output power will already be dropping, but the limitation makes the circuit more stable, though it will probably work without it.
It is likely that the 2pF capacitance will come directly from the PCB paths, and operational amplifiers are quite well compensated these days, so it is not really needed.
The 100R resistors serve in part to limit current and protect the operational amplifier in the event of a shorted output, but operational amplifiers have internal protection anyway.
They mainly allow the operational amplifier to control capacitive loads without oscillation.
A 2.2uF capacitor at the output blocks DC current, and the value is not particularly important.
This capacitor also forms a high-pass filter with a 10k potentiometer.
This application also provides reverse-polarity supply voltage protection in the form of a series rectifying diode.
