Electronics Types of Feedback in an Electronic Amplifier - Lab Report Example

3.1 Types of feedback There are 2 types of feedback in an electronic amplifier: 1. Negative feedback 2. Positive feedback Positive feedback In circuit amplifiers, a positive feedback can be described as a feedback in which the input signal, negative or positive is amplified and the positive output becomes more positive/negative becomes more negative and then some of the output voltage is returned back to the input by the feedback network. By some of the output adding to the input, the input becomes more positive/negative thus the input voltage becomes larger and larger until the output becomes saturated at its positive/negative supply rail. In short, in a positive feedback, more leads to more and less leads to less. This happens because the feedback is in phase with the input and thus set point and output values are added together. The transfer function for a positive feedback/if loop gain is positive for any system is given by: Av = G / (1 – GH) which implies that if GH=1, the Av is infinity and the circuit starts to oscillate itself after which no input signal is needed to maintain the oscillations (useful for oscillators). One major advantage of the positive feedback is that it makes transistors memorize commands. The other most precious advantage of positive feedback is that it enables the electronics to obtain a very fast switching response to a signal/condition. It mainly applies in areas where bi-stability is necessary. Below is a positive feedback block diagram. Negative feedback In the negative feedback control system, the feedback is out of phase with the original input thus the set point and output values are subtracted from each other. Its effect is to reduce the gain. This reduction results in stable circuits and increases the operating bandwidth of a given system. Below is a block diagram for negative feedback This feedback is achieved by when a small portion of the output signal Vout is put back to the inverting (-) input terminal using the feedback resistor RF. In this case therefore, due to the inversion, if Vin is positive, the op-amplifies the positive signal but the inversion makes the output to be negative. Some of the output voltage is returned back through the RF. The input is reduced by the negative feedback. The reverse is true for negative input. The output eventually settles down and becomes stabilized i.e more leads to less and less leads to more. In this scenario, the transfer function is given by Av= G/(1+GH). The greatest advantage of this feedback is that it leads to system stability. It also makes control systems to be more accommodating to random variations in component inputs. 3.1b Open loop gain G G=Vout/Vin Calculations G=25dB G=20dB after negative feedback (a) Required feedback factor (25x3)dB= 75dB 75dB=20log (G) G=antilog 1075/20 = 5620.76 3.1.b) Feedback factor is 10= 5620.76/1+B(5620.76) B=0.0998 3.2 (a) Ao = 140dB = 10000000 30dB = 31.62 =32 Ac=Ao/1+BAo Since A O>>1 SO taking Ao common from the denominator we get Ac= 1/(1/Ao+B) And 1/Ao is approximately zero, so we get Ac=1/B And B=1/Ac So B= 0.03125 For an inserting amplifier, the equation for B is given as B=Rin/Rin+Rf Where Rin is the input resistor and Rf is the feedback resistor. Using the value for B evaluated above we get 0.03215=4000/4000+Rf We get Rf= 120528.77 Where Rf=12.05 KΩ Input resistance with feedback Rinf=RiRf/RiR+Rf Rinf=3.004KΩ 3.3 Investigating the effects of applying feedback to single and multi-stage circuits a) simulation without feedback Input Frequency Output Voltage Gain 100 1.665 1.665 500 1.669 1.669 1000 1.671 1.671 5000 1.665 1.665 10000 1.665 1.665 50000 1.662 1.662 100000 1.662 1.662 1000000 1.664 1.664 b. When capacitors/ simulation with feedback Input Frequency Output Voltage Gain 100 1.838 1.838 500 1.838 1.838 1000 1.838 1.838 5000 1.563 1.563 10000 1.589 1.589 50000 1.552 1.552 100000 1.993 1.993 1000000 1.492 1.492 As can be seen, for lower frequencies in simulation without feedback, gain is higher for lower frequencies and changes significantly with changes in input frequency. At very high input frequencies, changes in gain are less significant. However, with feedback actuated, gains are unchanged for lower frequencies which then lowers in an oscillatory pattern i.e there is an oscillation in simulation with feedback but negligibly small oscillations in the simulation without feedback stabilizes. 5. Using a simple sinusoidal circuit, describe the basic conditions and method to achieve sinusoidal oscillation Oscillators do not require external input. This means that oscillations are produced by the method of positive feedback meaning the oscillator output feeds its own input. For a sinusoidal circuit, two conditions/methods are used to sustain a sinusoidal oscillation. The two conditions below should be fulfilled: i. Positive feedback occurs only at one frequency which is the required frequency of oscillation. This condition is fulfilled by ensuring that only signals of the desired frequency are fed back into the input or by ensuring that the feedback frequency is in correct phase at only one frequency ii. Sufficient amplification occurs only at the required frequency. This is attained by using an amplifier with an extremely narrow bandwidth that extends to the frequency of oscillation only. Below is a simple example of sinusoidal circuit 6. Types of Oscillators Phase-shift A phase-shift oscillator consists of a negative gain amplifier (-K) with a third order RC ladder network in the feedback. The circuit oscillates at the frequency for which the phase shift of the RC network is 180o. it’s only at this frequency that the total phase shift around the loop is 0o or 360o Tuned oscillators A tuned oscillator is a circuit which generates a radio frequency output by the use of a resonant circuit. Due to high frequencies, small inductance is used for the radio frequency of oscillation. 9. Explain and discuss the advantages of crystal oscillator compared to the rest of oscillating circuits. A crystal oscillator is an electronic oscillator circuit, which uses mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency. Quartz. The major advantage of quartz crystal oscillators is their high Q factor. A higher q factor indicates lower rates of energy loss as compared to the stored energy thus it has the lowest rate of dumping rate and oscillation is sustained longer than in LC oscillators. One other very important aspect about the quartz is that they exhibit very low phase noise. For other oscillators, spectral energy at resonant frequency is amplified and results in a collection of tones at different phases. For crystal vibrator, the crystal vibrates in one axis thus only one phase is dominant. This makes them the most stable and mostly used in telecommunications where stability is necessary. The quartz crystal may be affected by environmental not as much as other oscillators. It’s easy to design oscillators whose frequency is not easily affected by the temperature changes. The frequency of the oscillators does not fast respond to changes in vibration thus there are designs that reduce the environmental effects. This puts the crystal oscillators above most oscillators when it comes to stability. Read More