A warm greeting to my great friends and readers. In my last post, I discussed about the simple amplifier, maximum power theorem and its derivation, properties of operational amplifiers and their uses. You can read up more about them here. So, today I will be taking it up from where I last stopped by discusssing the non-inverting amplifier, the voltage follower, the comparator, and radio reception.
The non-inverting amplifier
The op-amp can be used to make an amplifier that does not invert the input signal. An example of such circuit is shown in the figure below. This circuit also operates so that the difference between the two inputs is so close to zero that it can be taken as zero.
This time, neither input is connected to earth (0 V). If the input to the non-inverting input is Vin then we assume that the voltage at the inverting input is also Vin. R1 and R2 form a potential divider. The voltage at the inverting input V(-) of the image shown is given by:
V(-) = R1/(R1 + R2) × Vout
This is equal to the input voltage, Vin so that:
Vin = R1/(R1 + R2) × Vout
The gain is thus:
Vout/Vin = (R1 + R2)/ R1 = 1 + R2/R1
Again, this is the closed loop gain and it depends only on the two resistors. There is one version of the non-inverting amplifier that is particularly useful. This is the voltage follower.
The voltage follower
In an illustration of a simple version of the non-inverting amplifier. The output is connected directly to the inverting input, which means that R2 is zero. The other resistor, R1, is removed. This is equivalent to making its size infinite. Therefore:
Gain = 1 + 0/0 = 1 + 0 = 1
So what good is an amplifier with a gain of 1?
The advantage of this circuit lies in the basic properties of the op-amp. The input draws a very small current and the output can provide a much larger current. The technical explanation is: ‘It has a very high input impedance and a very low output impedance. The following example illustrates the use of the voltage follower.
In a voltmeter connected to measure the p.d. of 1 V across a 1 MΩ resistor. A typical moving-coil voltmeter might have a resistance of about 100 kΩ, so this is not a very good way of measuring the p.d. since the voltmeter will draw most of the current and give an incorrect reading. A better way of measuring the p.d. is by using a voltage follower. The op-amp input draws far less current than the voltmeter, but the output can provide enough current for the voltmeter. A typical op-amp will draw about 1 pA with a voltage of 1 V at the input, but will provide several milliamps at the output. The voltage follower is a very useful little circuit and has some very interesting applications in physics.
The comparator
The op-am can be used as a switch. There is no feedback, so the two inputs do not have to be the same. The output depends on the difference between the two inputs. An op-amp with an open loop gain of 106 has a voltage of 13 μV at the non-inverting input. (The inverting input is held at 0 V.) The output will be at +13 V, the saturation voltage.
If the input is reduced by 13 μV to zero, the output also becomes zero. If the input is further reduced -13 μV, the output shoots down to -13 V. A change of 26 μV (0.000 026 V) is enough to switch the output of the op-amp from positive saturation to negative saturation. The op-amp can also be used as a voltage comparator. In an illustration of a typical application of this. The thermistor and the fixed resistor, R, form a potential divider, which fixes the voltage at the non-inverting input of the op-amp. As the temperature rises, this voltage increases as the resistance of the thermistor increases. (If the temperature falls, the voltage will decrease.) The voltage at the inverting input is set by the variable resistor which is used as a potentiometer.
The potentiometer is set so that the voltage at the inverting input is the same as the voltage at the non-inverting input. The difference between the two voltages will then be zero and the lamp at the output will be off. As soon as the temperature increases, the voltage difference will be greater than zero and the lamp will light. Comparators are very sensitive switches. The temperature need only change by a very small amount before the voltage difference exceeds 26 μV.
RADIO RECEPTION
I shall finish this series by looking at a simple radio designed to receive amplitude modulated signals. A transmitter adds an audio frequency signal to a radio frequency carrier to produce the amplitude modulated signal which is transmitted. A radio receiver receives this signal and carries out the reverse process, eventually ‘un-mixing’ the radio signal to produce the original audio frequency signal. The radio signal is transmitted as an electromagnetic wave and the signal is ‘picked up’ by an aerial. A length of wire makes a very effective simple aerial: many radios use a telescopic metal rod.
ELECTROMAGNETIC WAVES AND AERIALS
Electromagnetic waves have an electric field component and a magnetic field component. Different kinds of aerial respond to one or other of these components. In the presence of a radio signal, free electrons in a wire or telescopie aerial oscillate to produce an alternating current at the same frequency as the radio signal. The oscillation of the electrons in the wire aerial is induced by the electric field component of the radio wvave.
Most radios designed to receive medium and long wave signals use a ferrite rod aerial, which consists of a coil of wire wound on a ferrite rod. This aerial responds to the magnetic field component of the radio wave. The changing magnetic field induces a current in the coil. The ferrite rod concentrates the local field, making it stronger inside the coil.
The problem is that there are nmany radio signals, each with a different frequency and each inducing a current of the same frequency as the radio wave. The aerial carries all these alternating currents into the radio. The tuned circuit selects one signal out of the many present in the aerial. The tuned circuit is a parallel LC circuit. That is, it is a capacitor in parallel with an inductor. Most tuned circuits use a variable capacitor so that the circuit can be ‘tuned’ to cover a range of frequencies. The parallel LC circuit has a resonant frequency.
It the resonant frequency matches a signal from the aerial, this signal will be passed on. The tuned circuit behaves like a filter, blocking some signals and allowing a narrow range of frequencies through. The ‘ideal’ tuned circuit allows only one signal to pass through, so that the signal produced by the radio is free from interference from other signals. However, the tuned circuit must allow a band of frequencies through that is equal to the bandwidth of the AM signal.
Simple radios also need a good earth wire in contact with the ground to allow unwanted signals to be carried away. Note that this literally means the earth, not the mains power supply earth. The next stage is the ‘un-mixing’ or demodulation of the selected signal. A very simple and effective demodulator can be made using a single diode and a capacitor. The diode actually rectifies the signal, that is, it allows current in only one direction. The rectified signal still contains the lower frequency signal and the radio frequeney carrier.
The capacitor is chosen so that its reactance is low at the frequency of the radio signal. The radio frequency part of the rectified signal then passes through the capacitor to earth. The lower, audio frequency part of the rectified signal is not strong enough to drive a loudspeaker, but it can be amplified by an audio amplifier.
As its name suggests, the audio amplifier is designed to amplify audio frequency signals. A good amplifier should amplify equally over the whole of the audio frequency range (20 Hz to 20 kHz), although for most simple radios a much narrower range still gives acceptable results.
SUMMARY
After reading this chapter and the previous ones such as PHYSICAL ELECTRONICS and PHYSICAL ELECTRONICS: The Simple Amplifier, you should:
- Know that any electronic system can be broken down into three parts: input, process and output.
- Understand the terms transducer and sensor.
- Know that there are two kinds of electronic system: digital and analogue.
- Understand the terms op-amp, inverting amplifier and non-inverting amplifier.
- Understand the principles of radio reception.
REFERENCES
https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php
https://www.electronics-tutorials.ws/opamp/opamp_3.html
https://www.electronicshub.org/non-inverting-operational-amplifiers/#:~:text=A%20non%2Dinverting%20amplifier%20is,like%20a%20voltage%20follower%20circuit.
https://www.allaboutcircuits.com/video-tutorials/op-amp-applications-voltage-follower/
https://www.electrical4u.com/voltage-follower/
http://www.learningaboutelectronics.com/Articles/Voltage-follower#:~:text=A%20voltage%20follower%20
https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html
https://en.wikipedia.org/wiki/Comparator
https://www.acma.gov.au/radio-reception-and-interference
https://en.wikipedia.org/wiki/Radio_receiver
https://link.springer.com/chapter/10.1007/978-3-319-07806-9_18
https://www.explainthatstuff.com/antennas.html
https://courses.lumenlearning.com/physics/chapter/24-2-production-of-electromagnetic-waves/
https://en.wikipedia.org/wiki/Antenna_