[Photo] Broadband high frequency power amplifier

Same. For ordinary transformers, the signal voltage is applied to the first and second ends of the primary winding, so that the primary coil has a current flow, and then through the magnetic field line, the corresponding alternating voltage is induced at the secondary 3 and 4 ends, and the energy is transferred from the primary To the secondary load. However, the signal voltage of the transmission line is applied to the terminals 1 and 3, and the energy propagates in the medium between the two wires, and reaches the load on the output from the input.

For the transmission line, it can be regarded as a coupling chain composed of many inductances and capacitances, which is at the bottom, as shown in Figure 3-26. The inductance is the inductance of the wire ΔL, and the capacitance is the distributed capacitance between the two wires.

When the signal source is added to terminals 1, 3, due to the presence of the capacitance between the transmission lines, the signal source charges the electricity and the capacitance, so that the capacitor stores the electric field energy. The capacitor discharges near the inductor to make the inductor store magnetic field energy. That is, electric field energy can be converted into magnetic field energy. Then the inductance exchanges energy to the capacitor behind, and the magnetic field energy is converted into electric field energy. Then, the capacitor exchanges energy with the inductance in the back, and so on. The input signal is transmitted in the form of electromagnetic energy exchange from the beginning to the terminal, and finally absorbed by the load.

In the transmission line transformer, the distributed capacitance between the lines is not an unfavorable factor affecting high-frequency energy transmission, but rather an indispensable condition for electromagnetic energy conversion. In addition, electromagnetic waves mainly propagate in the medium between the wires, so the loss of the magnetic core greatly reduces the signal transmission. The maximum operating frequency of the transmission line transformer can be greatly improved, so as to achieve the purpose of broadband transmission. Strictly speaking, transmission line transformers transmit energy in different ways at high and low frequency bands. At high frequencies, it is mainly transmitted by means of electromagnetic energy transformation. At low frequencies, it will be transmitted simultaneously through the transmission line and magnetic coupling. The lower the frequency, the worse the efficiency of the transmission line to transmit energy, and the more it depends on the magnetic coupling method for transmission.

Second, the broadband transmission line transformer circuit

(1) 1: 1 transmission line transformer

The transmission line transformer shown in Figure 3-25 is called a 1: 1 transmission line transformer, also known as an inverter transformer. According to the transmission line theory, when the transmission line is a lossless transmission line and the load impedance RL is equal to the transmission line characteristic impedance Zc, the relationship between the transmission line terminal voltage U2 and the starting terminal voltage Ul is ←

U 2 = Ul e -jωt

In the formula, α = 2π / λ is the phase shift constant of the transmission line, the unit is rad / m. It is the operating wavelength, and t is the length of the transmission line. If the length of the transmission line is very short, satisfying αl <1, then e-jωt≈1, then U 2 = U 1, that is, the amplitude of the voltage U of the input end of the transmission line is equal to the voltage U 2 of the output end, and the phase is approximately the same. In the same way, I 2 = I le -jωt and I 1 = I 2 is necessary. Under the condition that the 2 and 3 terminals are grounded, a voltage with the same amplitude and opposite phase as the input terminal is obtained on the load RL, that is

UL = -UI

It can be seen from the circuit diagram that the conditions for matching transformer and fu are:

Zc = RL,

The conditions for matching the signal source to the transmission line transformer are

ZC = Rs

Obviously, the best matching condition for 1: 1 transmission line transformers is

Zc = Rs = RL

The power obtained on the load RL is

Po = I 2 RL

And I l = I 2, then

Po = I 2 RL = [Us / (Rs + Zc)] 2R L

Under the condition of RL = Zc = Rs, the maximum power can be obtained on RL.
In various amplifying circuits, it is rare that RL is exactly equal to the internal resistance of the signal source. Therefore, 1: 1 transmission line transformers are more used as inverters

(2) 1: 4 impedance transformation transmission line transformer 1: 4 transmission line transformer. It can function as a 1: 4 impedance converter, that is, Rs: RL = 1: 4. The following only illustrates the optimal matching conditions and the relationship of impedance transformation on the relationship between the voltage and current of the ideal, lossless transmission line.

Since the lossless transmission line is under matching conditions, U l = U 2 and I 1 = I 2, then

Z i = U 1 / I 1 + I 2 = U 1 / 2I 1 = Zc / 2

In addition

RL = U 1 + U 2 / I 2 = 2U 1 / I 1 = 2Z C

Therefore, under the best matching conditions, the transmission line transformer Rs = Zi = Zc / 2 = RL / 4 is equivalent to a 1: 4 impedance converter.

(3) 4: 1 impedance transformation transmission line transformer

According to the master of 4: 1 impedance transformation, the circuit shown in Figure 3-28 can be used to form.

Below we still use the relationship between the voltage and current of the ideal element loss transmission line to illustrate the optimal matching conditions and the relationship of impedance transformation.

Since the lossless transmission line is under matching conditions, U 1 = U 2 and I 1 = I 2

Z i = UI + U 2 / I 1 = 2U 1 / I 1 = 2Zc

In addition,

RL = U 2 / I 1 + I 2 = U 1 / 2I 1 = 1/2 Zc

Therefore, under the best matching conditions,

Rs = Zi = 2Zc = 4R L.

3. Broadband high frequency power amplifier

A broadband high-frequency power amplifier drawn by a transmission line transformer and a transistor uses a transmission line transformer to transmit high-frequency energy in a wide frequency band and achieve impedance matching between the amplifier and the amplifier or between the amplifier and the load. Figure 3-29 is a typical circuit of this power amplifier.

B1, B2 and B3 are wide-band transmission line transformers. Bl and B2 are connected in series to form a 16: 1 impedance converter to match the high output impedance of Tl with the low input impedance of T2. Each stage of the circuit uses a negative voltage feedback circuit to improve the performance of the amplifier. Resistors 1.8KΩ and 47Ω connected in series provide feedback to the T1 amplifier, and resistors 1.2KΩ and 12Ω connected in series provide feedback to the T2 amplifier. In order to avoid the parasitic coupling between the amplifier stages through the internal resistance of the power supply, the RC decoupling filter circuit is used. The filter capacitor is composed of three capacitors of different sizes connected in parallel to filter different frequencies. Since no tuning loop is used, this amplifier should work in Class A state. Class B push-pull circuits are used for the input to improve efficiency.

The operating frequency range of this circuit is (2-30) MHZ, and the output power is 6OW. According to the load of 50Ω, after the 4: 1 impedance transformation of B3, the collector load of T2 is 200Ω. Since it works in a high power state, its input resistance is about 12Ω, and it will change with the size of the input signal. In order to reduce the influence of the input impedance change on the pre-amplifier, a 12Ω resistor is connected in parallel to the input end of T2, so that the total input resistance becomes 6Ω. After the 16: 1 impedance transformation, the collector load of T1 is 96Ω .