JPH0754886B2 - Power amplifier - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a high frequency power amplifier using a λ / 4 transmission line for a bias circuit.

(Prior Art and Problems) FIG. 1 is a diagram showing an example of a conventional power amplifier, showing a configuration example using a micro strip line.
Is an input terminal, 2 is an output terminal, 3 is a semiconductor device, 4 and 5 are λ / 4 transmission lines, 6 and 7 are coils, 8 and 9 are capacitors,
10 and 11 are capacitors for suppressing power supply ripple, 12, 13
Is a bias voltage application terminal, 14 and 15 are DC cut capacitors, 16 is an input matching circuit, 17 is an output matching circuit, and 18 is an output terminal (collector or drain) of the semiconductor device.

The λ / 4 transmission lines 4 and 5 have a length of λ / 4 with respect to the signal frequency, and their ends are short-circuited at high frequencies by capacitors 8 and 9. Therefore, the impedance becomes extremely large at the signal frequency, and does not affect the input matching circuit 16 or the output matching circuit 17 at all. This λ / 4 transmission line 4 or 5 is conventionally connected directly to the output terminal (collector or drain) or input terminal (base or gate) of the semiconductor device as shown in FIG. 1 or to the transmission line of the matching circuit. ing.

Generally, it is desired that the power amplifier has low power consumption, that is, high efficiency, but the efficiency of the power amplifier is 7 in class B operation.
8.5% is the theoretical maximum efficiency, in practice 40-5 at 1 GHz
It is about 0%. In class C operation, an efficiency of about 60% can be obtained at 1 GHz, while its ideal efficiency reaches 100%, but at this time, the output power is zero, which is impractical.
According to the reference, the output circuit of the semiconductor device has the conventional matching condition for the signal frequency, and the impedance is zero for even harmonics and infinite for odd harmonics. It has been shown that 100% efficiency is ideally obtained even in class operation. Furthermore, even if the impedance is zero for even harmonics,
It has been shown that "optimal efficiency" is obtained.

In order to realize an output circuit having such an impedance, the circuit shape must be reduced without changing the impedance of the output circuit at the signal frequency,
Furthermore, since it is necessary to design a circuit in consideration of parasitic elements of the semiconductor device at high frequencies, there is a problem that the design is very difficult.

(Reference) SNIDER.D “A Theoretical Analysis and Experimental
Confirmation of the Optimally Loaded Overdriven RF
Power Amplifier ", IEEE Trans., 1976 ED-14, no.12, p8
(Object of the Invention) The present invention optimizes the connection position of a bias circuit using a λ / 4 transmission line in order to satisfy the condition of short for even-order harmonics. The purpose is to improve efficiency. Hereinafter, the configuration and operation of the present invention will be described based on examples.

(Embodiment of the Invention) FIG. 2 shows an embodiment of the present invention. 2 is an output end, 3 is a semiconductor device, 5 is a λ / 4 transmission line, 6 is a coil, and 8 is a signal frequency or more. A capacitor, 13 is a bias voltage application terminal, 15 is a DC blocking capacitor, 17 is an output matching circuit, 18 is an output end of a semiconductor device, and 19 is a connection point between the output matching circuit 17 and the λ / 4 transmission line 5. .

Since the λ / 4 transmission line 5 is terminated by the capacitor 8, it becomes a short circuit in terms of high frequency. Therefore, the impedance Z (n) viewed from the connection point 19 to the bias voltage application terminal 13 side is represented by Z (n) = jZ ₀ tan βn · l (1). Here, n is the order of harmonics having the signal frequency as the fundamental wave, Z ₀ is the characteristic impedance of the transmission line, and β n is n
The phase constant for the next harmonic, l is the length of the λ / 4 transmission line 5 and is λ / 4. At this time, Z (n) is Becomes By connecting the λ / 4 transmission line 5 to the semiconductor device 3 and the output matching circuit 17 as a bias circuit, the condition that the impedance becomes zero for even harmonics is a connection point.
Fill with 19. Since the semiconductor device has a parasitic element at a high frequency, even if the transmission line 5 is connected to the output end 18 of the semiconductor device 3, high efficiency cannot be obtained. The harmonic impedance, which is a condition for high efficiency, is for the current source of the semiconductor device. Therefore, it is necessary to connect the bias circuit in consideration of the parasitic element of the semiconductor device. Since the parasitic element of the semiconductor device has only a reactance component, it can be equivalently regarded as a transmission line. Since the impedance of the current source is infinite, the reactance component of the semiconductor device at a certain frequency can be obtained from the output impedance at that frequency. The output impedance Z of the semiconductor device is Z
= R + jX (Ω), the equivalent transmission line l _t in the semiconductor device has a characteristic impedance of Z ₀ and a signal frequency wavelength of λ ₁ . Indicated by. Considering this l _t , the condition of the output circuit for obtaining zero impedance when the output circuit is viewed from the current source of the semiconductor device is obtained.

Assuming that the equivalent transmission line l _t in the semiconductor device is the length l _x from the output end 18 of the semiconductor device to the connection point 19, the connection point 19 is short-circuited as described above in the even-ordered semiconductor device. The even-order impedance Z _S (n) viewed from the current source is Z _S (n) = jZ ₀ · tanβn (l _t + l _x ) (4). To have Z _S (n) = 0 when n is an even number, at least To get From equations (3) and (5) To get By setting the distance from the connection point 19 of the transmission line 5 to the output end 18 of the semiconductor device 3 to l _x that satisfies the equation (6), when the output circuit is seen from the current source of the semiconductor device, the impedance is even It becomes zero. X in the equation (6) is a reactance component of the output impedance of the semiconductor device measured at the center frequency ₀ of the used band of the power amplifier. l _x is a frequency with at least the band used in equation (6)
If the condition is satisfied, the even-order impedance becomes zero in the band, and the efficiency of the power amplifier is improved. Further, at the signal frequency, the impedance seen from the connection point 19 to the λ / 4 transmission line 5 becomes infinite and has no influence on the output matching circuit 17.

(Effects of the Invention) As described in detail above, the power amplifier of the present invention is
A bias circuit using a λ / 4 transmission line is connected to an output matching circuit in consideration of parasitic elements of a semiconductor device,
There is an advantage that a highly efficient power amplifier can be realized with the same scale and simple structure as the conventional circuit.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a conventional power amplifier, and FIG. 2 is a diagram showing one embodiment of the present invention. 1 ... input terminal, 2 ... output terminal, 3 ... semiconductor device,
4, 5 ... λ / 4 transmission line, 6, 7 ... Coil, 8, 9 ...
… Capacitors, 10, 11 …… Capacitors for suppressing ripple of power supply, 12, 13 …… Bias voltage application terminals, 14, 15
...... Capacitor for DC cut, 16 …… Input matching circuit,
17 ... Output matching circuit, 18 ... Output terminal of semiconductor device, 19 ...
... Connection point between output matching circuit 17 and λ / 4 transmission line 5

Claims (1)

[Claims] 1. An output end of a semiconductor device is connected to an output matching circuit formed by a microstrip line, one end of which is connected to a position of a distance l from the connection point on the output matching circuit, and the other end of which is connected. A power amplifier having a bias circuit configured to connect a power source to a side of a λ / 4 transmission line terminated by a capacitor to supply a DC voltage to the semiconductor device, When the output impedance of the semiconductor device at the center frequency of the band is R + jX (Ω), the used wavelength is λ, the characteristic impedance of the bias circuit is Z ₀ , and n is a natural number, the λ / 4 transmission line is connected to the semiconductor device output terminal. And the distance l from the connection point between the output matching circuit and A power amplifier connected to a corresponding position on an output matching circuit so as to obtain a value obtained by.