What Is a Tuned Amplifier?

Tuning amplifiers refer to amplifiers that use a resonant circuit as a load, that is, a circuit composed of a capacitor and an inductor as a load, and the gain and load impedance vary with frequency. When its center frequency can be arbitrarily adjusted within a certain frequency range, it can be achieved by mechanical tuning (variable capacitor or variable inductor) or electrical tuning (varactor). ^[1]

Chinese name: Tuning amplifier

Foreign name: tuned amplifier
Features: Signal amplification

Introduction to Tuning Amplifiers

With a capacitor and inductor as the load, the gain and load impedance change with frequency

Tuning amplifier Amplifier circuit. This loop is usually tuned to the center frequency of the signal to be amplified. Because the parallel resonance impedance of the tuning loop has a large value near the resonance frequency, the amplifier can obtain a large voltage gain. At frequencies farther away from the resonance point, the loop impedance drops rapidly, causing the amplifier gain to decrease rapidly; therefore, a tuning amplifier is usually a narrowband amplifier with high gain and good frequency selectivity.

Main quality index of tuning amplifier

Measuring the main quality of a tuning amplifier includes the following aspects:

Tuning amplifier resonance frequency

The frequency corresponding to the resonance of the amplifier tuning loop is called the resonance frequency of the amplifier. In theory, for a parallel resonant circuit composed of LC, the expression of the resonance frequency is:

In the formula, L is the inductance of the tuning loop inductance coil; C is the total capacitance of the tuning loop.

Av Tuning amplifier resonance gain (Av)

The amplifier's resonance voltage gain magnification refers to the ratio of the output voltage to the input voltage at the resonance frequency f0.

Av measurement method: When the resonance circuit is in a resonance state, use a high-frequency millivolt meter to measure the input signal Vi and the output signal Vo, and use the following formula to calculate:

In addition, you can also use the power gain coefficient to estimate:

Tuning amplifier passband

Due to the frequency selection of the resonance circuit, when the operating frequency deviates from the resonance frequency, the voltage amplification factor of the amplifier decreases. It is customary to refer to the frequency deviation when the voltage amplification factor Av = Vo / Vi drops to 0.707 times the resonance voltage amplification factor Avo Shift is called the passband bandwidth BW of the amplifier, which is usually expressed by 2f0.1, sometimes also called 2f0.1 as the 3dB bandwidth. Passband bandwidth:

Where Q is the on-load figure of merit of the resonant circuit.

When the total capacitance of the loop is fixed after the transistor is selected, the product of the resonance voltage amplification factor fo and the passband BW is a constant.

Measurement method of band BW: According to the concept, the pass band can be obtained by measuring the resonance curve of the amplifier. The measurement method mainly uses the frequency sweep method, which can also be a point-by-point method.

Frequency sweep method: that is, directly test with a frequency sweeper. During the test, the output of the frequency sweeper is connected to the input of the amplifier, the output of the amplifier is connected to the input of the detector of the frequency sweeper, and the output of the detector is connected to the input of the scanner. Observe and record the frequency characteristic curve of the amplifier on the frequency sweeper, and read and record the passband of the amplifier from the curve.

Point-by-point method: It is also called point-by-point measurement method, which is to measure the corresponding signal size of the circuit at different frequency points, and use the obtained data to make a curve of the signal size as a function of frequency. frequency band.

The specific measurement method is as follows:

a. Use an external dedicated signal source as the sweep source. Select the proper amplitude of the sinusoidal input signal and keep it unchanged.

b. The oscilloscope monitors the input and output waveforms at the same time to ensure that the circuit works normally (the circuit has no interference, no self-excitation, and output

Waveform without distortion);

c. Change the frequency of the input signal and use a millivolt meter to measure the effective value of the output voltage at different frequencies;

d. Draw the frequency characteristic curve of the amplifier, and read and record the passband of the amplifier on the frequency characteristic curve. When testing, you can first tune the resonance circuit of the amplifier to make it resonate, note down the resonance frequency fo and the voltage amplification factor Avo at this time, and then change the frequency of the high-frequency signal generator (keep its output voltage unchanged) and measure Voltage magnification. The resonance curve of the amplifier is shown in Figure 1-1 because the voltage amplification factor of the loop is detuned.

Amplifier passband and resonance curve

Tuning Amplifier Gain Bandwidth Product

The gain-bandwidth product BW · G is also an important index for communication electronic circuits. Generally, the gain-bandwidth product can be considered as a constant. The total passband width of the amplifier becomes narrower as the number of amplification stages increases. The larger the BW, the smaller the gain. The two are a contradiction.

In different circuits, the passband difference of the amplifier may be relatively large. For example, when designing IF amplifiers for televisions and radios, bandwidth considerations are different. The bandwidth of an ordinary AM radio broadcast is 9kHz, while the bandwidth of a TV signal requires 6.5MHz. Obviously, to obtain the same gain, The bandwidth design of amplifiers is completely different.

Tuning amplifier selectivity

The ability of an amplifier to select useful signals from the sum of signals of various frequencies and exclude interference signals is called the selectivity of the amplifier. The basic indicator of selectivity is the rectangular coefficient. Among them, the defined rectangular coefficient is the ratio of the frequency offset corresponding to the voltage amplification factor falling to 10% of the amplification factor at resonance and the frequency offset 2f0.1 corresponding to the voltage amplification factor falling to 0.707, namely:

It is also possible to define rectangular coefficients, that is:

Obviously, the closer the rectangular coefficient is to 1, the closer the curve is to a rectangle, the stronger the ability to filter out interference signals from adjacent channels.

Tuning amplifier application range

Tuning amplifiers are widely used in high-frequency amplifier stages of various radio transmitters and high-frequency and intermediate-frequency amplifiers in receivers.

Tuning amplifier Great. There are always a variety of electromagnetic waves in space at the same time. All we need to receive is the useful signals we are interested in, while other unwanted electromagnetic waves are interference to the receiver. How to more effectively select signals and suppress interference is One of the important tasks of the receiver. Therefore, tuning amplifiers are widely used in receivers. Such amplifiers have a large amplification factor for signals near the tuning frequency, and have a smaller amplification factor for signals farther away from the tuning frequency, or even attenuate it. ^[2] In the receiver, it is mainly used to amplify the voltage of small signals, so most of them work in the state of Class A amplification. In the transmitter, it is mainly used to amplify radio frequency power, so it mostly works in the C or B state (see power amplifier).

The tuning circuit of the tuning amplifier may be a single tuning circuit or a dual tuning circuit in which two circuits are coupled. It can be coupled to the next stage through mutual inductance, or it can be coupled to the next stage through capacitance. Generally speaking, the frequency response of an amplifier using a dual tuning loop is in the passband.

Tuning amplifier The inside can be made flatter with a steeper cutoff at the edge of the band. The IF amplifier in a superheterodyne receiver often uses a dual-loop tuning amplifier. The product of the gain and bandwidth of a single-stage tuning amplifier is limited by the parameters of the amplifier device. When the device is selected, the higher the gain of the amplifier, the narrower the bandwidth. To ensure sufficient gain and proper bandwidth, several stages of tuned amplifiers are often cascaded. Sometimes the loop of a two-stage (or three-stage) amplifier is tuned to two (or three) different frequencies respectively to form a staggered tuning amplifier. This amplifier has a wider frequency band and higher total gain, but the adjustment of the amplifier is more troublesome. This type of amplifier is often used in intermediate frequency amplifier stages of radar receivers.

The stray parameters of the amplifier device have an effect on the performance of the tuning amplifier. For example, due to the feedback effect of the transistor collector junction capacitance C C, the amplifier operation may be unstable, and even self-excited oscillation may occur. It can usually be eliminated by neutralization. Figure 3 shows a tuning amplifier with a neutralization circuit. CN is a neutralization capacitor. The output signal is fed back to the input of the amplifier by the loop inductance L through CN to cancel the internal feedback of the inter-electrode capacitance C C.