Double-Tuned Voltage Amplifier

DOUBLE-TUNED VOLTAGE AMPLIFIER

To overcome the drawback of single tuned amplifier, double tuned amplifiers are used. In this method, two tuned circuits are inductively coupled to each other. Fig.3.33 shows the circuit of a double tuned voltage amplifier.

L₁ C₁ is used as collector load and L₂ C₂ is used as output circuit. R₁, R₂ R_E provide dc current and voltage for the transistor.

This circuit provides high impedance to the input signal and a large output appears across the tuned circuit L₁ C₁. This output is inductively coupled to L₂ C₂ tuned circuit.

This is used in radio and television receiver. The Fig. 3.34 shows the frequency response of double tuned amplifier.

This circuit provides high selectivity, high gain and large bandwidth.

STAGGER-TUNED AMPLIFIER

If two or more circuits are synchronously tuned and cascaded, the overall bandwidth decreases.

In stagger tuned amplifiers, the cascaded circuits are tuned to slightly different frequencies, then bandwidth will be increased. This technique is called stagger tuning.

The tuned circuits are tuned to slightly different frequencies. The frequency response of the stagger tuned amplifier is shown in Fig. 3.35.

f₁ - resonant frequency of L₁ C₁ resonant circuit

f₂ - resonant frequency of L₂ C₂ resonant circuit

f_O - resonant frequency of stagger tuned circuit

This amplifier has a greater bandwidth and flatter passband, if more number of stages are used.

SOLVED PROBLEMS

Problem 3.1

A parallel resonant circuit has an inductance of 150 μH and a capacitor of 100 pF. Find the resonant frequency.

Given:

L = 150 μH, C = 100 pF

Solution:

We know that, Resonant frequency, 

Problem 3.2

Calculate the bandwidth between the half-power points of circuit which resonates at 1 MHz frequency and has Q of 100.

Given:

f_O = 1 MHz, Q_O = 100

Solution:

Problem 3.3

A tuned amplifier has its maximum gain at a frequency 2 MHz and bandwidth of 50 KHz. Find the Q-factor.

Given:

f_O = 2 MHz, BW = 50 KHz

Solution:

We know that

Table 3.2. Comparison of Tuned Amplifiers

Advantages of Tuned Amplifiers

i. They amplify the desired frequencies

ii. Good SNR

iii. Suitable for radio transmitters and receivers

iv. The range of frequencies can be varied.

Disadvantages of Tuned Amplifiers

i. Not suitable for amplifying audio frequencies

ii. If band of frequency is increased, design becomes complex

iii. Circuit is bulky and expensive.