Table of Contents
Tuned Amplifier, Series Resonant circuit, Paralel Resonant circuit, Single tuned Amplifier, General shape of Frequency response of Amplifier
Communication circuit widely uses tuned Amplifier. And they are used in MW & SW radio Frequency 550 KHz – 16 MHz, 54 – 88 MHz, FM 88 – 108 MHz, cell phones 470 – 990 MHz
Band width is 3 dB Frequency interval of pass band and –30 dB frequency interval
Similar to power Amplifiers, tune Amplifiers are also Categorised as A, B, or C based on the Conduction angle of the devices.
Series Resonant circuit
Series resonant features minimum impedance (RS) at resonant.
f r = ½√LC; q = L/Rs at resonance L=1/c, BW=fr/Q
It behaves as purely resistance at resonance, capacitive below and inductive above resonance
Paralel resonant circuit
Paralel resonance features maximum impedance at resonance = L/RsC
At resonance Fr=1/2√1/(LC-Rs2/L2); if Rs=0, fr=1/2√(LC)
At resonance it exhibits pure resistance and below for parallel circuit exhibits inductive and above capacitive impedance
1. Need for tuned circuit
To understand tuned circuits, we first have to understand the phenomenon of self-induction. And to understand this, we need to know about induction. The first discovery about the interaction between electric current and magnetism was the realization that an electric current created a magnetic field around the conductor. It was then discovered that this effect could be enhanced greatly by winding the conductor into a coil. The effect proved to be two-way: If a conductor, maybe in the form of a coil was placed in a changing magnetic field, a current could be made to flow in it; this is called induction.
So imagine a coil, and imagine that we apply a voltage to it. As current starts to flow, a magnetic field is created. But this means that our coil is in a changing magnetic field, and this induces a current in the coil. The induced current runs contrary to the applied current, effectively diminishing it. We have discovered self-induction. What happens is that the self-induction delays the build-up of current in the coil, but eventually the current will reach its maximum and stabilize at a value only determined by the ohmic resistance in the coil and the voltage applied. We now have a steady current and a steady magnetic field. During the buildup of the field, energy was supplied to the coil, where did that energy go? It went into the magnetic field, and as long as the magnetic field exists, it will be stored there.
Now imagine that we remove the current source. The magnetic field begins to vanish in the absence of a constant current, but this places our coil once more in a variable field that induces a current into it. This time, the current is flowing in the same direction as the applied current, which prevents the magnetic field and current from decaying until all of the stored energy has been released. This can have a funny effect because the coil needs to release the energy it has been holding onto, so the voltage above it keeps rising until a current can flow somewhere! This means that when coils are involved, a surprising amount of sparks and arching can occur. You can actually receive an electric shock from a low-voltage source, such as an ohmmeter, if the coil is large enough.
2. Single tuned Amplifier
Single Tuned Amplifier have a single Tank Circuit, which determines the amplifying frequency range. by sending signals across a range of frequencies to the device’s input terminal. The amplified signal is completely available on the output terminal because the Tank Circuit’s collector delivers high impedance at resonant frequency. And because the tank circuit has a lower impedance for input signals other than resonant frequencies, the majority of the signals are attenuated at the collector terminal.
Ri- input resistance of the next stage
R0-output resistance of the generator gmVb‘e
Cc & CE are negligible small
The equivalent circuit is simplified by
General shape of Frequency response of Amplifier
A frequency amplifier for audio signals that operates between 20 Hz and 20 kHz. Radio receivers, large public gatherings, and various announcements made to passengers on railroad platforms all use audio frequency amplifiers. An amplifier should, in theory, provide the same amplification for all frequencies across the frequency range at which it is to be used. The curve known as the frequency response curve of the amplifier typically indicates how much of this is done.
To plot this curve, input voltage to the Amplifier is kept constant and Frequency of input signal is Continuously varied. The output voltage at each Frequency of input signal is noted and the gain of the Amplifier is Calculated. For an audio Frequency Amplifier, the Frequency range is quite large from 20 Hz to 20 kHz. In this Frequency response, the gain of the Amplifier remains constant in Mid-Frequency while the gain varies with Frequency in low and high Frequency regions of the curve. Only at low and high Frequency ends, gain Deviates from ideal Characteristics. The Decrease in voltage gain with Frequency is called Roll-off.
1. Definition of cut-off Frequencies and Bandwidth
The range of Frequencies can be Specified over which the gain does not deviate more than 70.7% of the maximum gain at some Reference Mid-frequency.
From above figure, the Frequencies f1 & f2 are called lower cut-off and upper cut-off Frequencies.
Bandwidth of the Amplifier is defined as the difference between f2 & f1.
Bandwidth of the Amplifier = f2 – f1
While Frequency f1 is in the low Frequency range, Frequency f2 is in the high Frequency range. Since gain or output voltage drops to 70.7% of its maximum value and this Corresponds to a power level of one half the power at the Reference Frequency in the Mid-Frequency region, these two Frequencies are also known as Half-power Frequencies.
Transistors are used in tuned RF amplifier to amplify a condensed band of high frequencies centred on a ratio frequency at frequencies closer to their unity gain bandwidths (i.e., fT). At this frequency, the transistor’s inter junction capacitance, abbreviated Cbc,. Which is the reactance between the base and collector and is otherwise infinite and best ignored as an open circuit, becomes dominant. Due to its CE configuration, the capacitance Cbe shown in Fig. 3.35 comes into contact with an amplifier’s input and output circuits. The feedback path from collector to base is provided because the Cbc’s reactance at RF is sufficiently low.
With this circuit condition, if some feedback signal manages to reach the input from output in a positive manner with proper phase shift, then there is possibility of circuit converted to a positive manner with proper phase shift, then there is possibility of circuit converted to an unstable one, generating its own oscillations and can stop working as an amplifier. This circuit will always oscillate. If enough energy is fed back from the collector to the base in the correct phase to overcome circuit losses. Unfortunately, the conditions for best gain and selectivity are also those which promote oscillation. In order to prevent oscillations in tuned RF amplifiers. It was necessary to reduce the stage gain to a level that ensured circuit stability. This could be accomplished in several ways such as lowering the Q of tune circuits; stager tuning, losse coupling
between the stages or Inserting a ‘loser’ element into the circuit. While all these methods reduced gain, Detuning and Q Reduction had Detrimental effects on Selectivity. Instead of loosing the circuit performance to achieve Stability, the professor L.A. Hazeltine introduced a circuit in which the Troublesome effect of the Collector to base Capacitance of the Transistor was Neutralized by Introducing a signal which cancels the signal coupled through the Collector to base Capacitance. He proved that the Neutralization can be Achieved by Deliberately feeding back a portion of the output signal to the input in such a way that it has the same Amplitude as the Unwanted Feedback but the opposite phase. Later on many Neutralizing Circuits were introduced. Let us study some of these circuits.
One type of Hazeline circuit is Depicted in fig. 3.36. In this circuit. Point B on the coil is connected to the base by a small amount of Variable Capacitance (CN). Therefore, a signal from the top end of the coil, point A, is fed to the Transistor base by the internal Capacitance Cbc, which is shown dotted, and a signal of equal Magnitude. But opposite Polarity is fed to the base from the bottom of the coil, point B, by the CN. The signal sent through the Cbc can be completely Eliminated by Correctly Adjusting the Neutralising Capacitor, CN.
Neutralization using coil
Using a coil, an RF amplifier is neutralised in Figure 3.38. In this circuit, the other winding is oriented with the least coupling to the L part of the tuned circuit at the base of the following stage. It is mounted at a 90-degree angle to the coupled windings and wound on a separate form. The voltage across L caused by the base circuit’s circulating current will have the correct phase to cancel the signal coupled through the base to collector capacitance, Cbc, if the windings are properly polarised.