JP3133864B2 - Transistor amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative feedback type transistor amplifier mainly used in a microwave band.

[0002]

2. Description of the Related Art As an amplification terminal in a microwave band, a homojunction or heterojunction bipolar transistor (hereinafter simply referred to as a transistor) formed on a semiconductor substrate such as silicon or gallium arsenide is often used. When wide bandwidth or stability is required for an amplifier, a circuit configuration of a negative feedback amplifier in which resistive feedback is applied between a base and a collector of a transistor is often adopted.

FIG. 4 shows a conventional circuit configuration of a one-stage resistive negative feedback transistor amplifier. Where 1
Reference numeral 9 denotes an amplifying transistor, and B, C, and E indicate a base, a collector, and an emitter of the transistor, respectively. Reference numeral 20 denotes a negative feedback resistor (Rf), and the feedback amount increases as the resistance value decreases. Reference numeral 21 denotes a DC separating capacitor (Cf) between the base and the collector, which normally has a capacitance so as to have a sufficiently small impedance in a frequency band of a target signal.
22, 22a are base bias resistors (Rb1, Rb
2) and 23 are collector bias resistors (Rc), 50 is a bypass capacitor (Cs1), Vbb is a base bias, and Vcc is a collector bias. An inductor may be used instead of the collector bias resistor Rc. The signal is input from the base terminal 24 and output from the collector terminal 25. In this circuit configuration, the amount of negative feedback from the collector to the base increases on the low-frequency side where the gain of the transistor is high, so that the gain of the amplifier is suppressed. On the other hand, on the high frequency side where the transistor gain is reduced, the amount of negative feedback is also reduced, so that the gain suppression of the amplifier is small. As a result, the gain of the amplifier has a flat characteristic over a wide band. In addition, since the above-mentioned feedback also has a function of suppressing amplitude fluctuation of a signal, it has an effect of suppressing oscillation of the amplifier and stabilizing the amplifier.

FIG. 5 shows a case where an amplifying transistor is connected to a stage subsequent to the conventional amplifier. Here, 26 transistors, 27 negative feedback resistance (Rf1), 28 DC separation capacitor (Cf1), 2
9, 29a base bias resistors (Rb3, Rb4),
The first stage negative feedback amplifier is constituted by the collector bias resistor (Rc1) of 30 and the bypass capacitor (Cs1) of 51.
35a base bias resistors (Rb5, Rb6), 36
Is connected to the collector bias resistor (Rc2).
Vbb indicates a base bias, and Vcc indicates a collector bias. Reference numeral 31 denotes a capacitor (Cc1) for separating the collector of the first-stage transistor and the base of the second-stage transistor in a DC manner. The signal is input from a terminal 37 and output from a terminal 38.

[0005]

However, the above-described conventional example has the following problems or room for improvement.

First, in the conventional example shown in FIG. 4, when the capacitance of the capacitor Cf is small and the capacitor Cf has an impedance that cannot be ignored with respect to the feedback resistor Rf in a target frequency band, Since the negative feedback on the low frequency side is suppressed by the impedance of the capacitor Cf, the suppression of the transistor gain on the low frequency side becomes insufficient and sufficient gain flatness cannot be obtained. On the other hand, if the capacitance of the capacitor Cf is increased so that the impedance of the capacitor Cf becomes sufficiently smaller than the resistance Rf, the size of the capacitor increases, which is disadvantageous for downsizing the circuit. Also, no matter how large the capacitance of the capacitor Cf, the gain of the transistor itself decreases as the frequency increases, so that an amplifier having a high gain on the high frequency side cannot be realized with the conventional circuit. However, when the amplifier is a component of a certain system, the flatness of the overall gain of the system is considered in consideration of an increase in transmission loss on the high frequency side due to wiring or the like connecting the components of the system. In many cases, it is desirable to have an amplifier that increases the gain on the high frequency side in order to achieve higher performance.

Next, in the conventional example shown in FIG. 5, a resistor 36 (Rc2), a resistor 30 (Rc1), a resistor 27 (Rf1) and a capacitor 28 are output from the output terminal 38 of the second stage amplifier.
Although a feedback loop via the bias line, which reaches the input terminal 37 of the amplifier via (Cf1), occurs, the transistors in each stage operate as anti-phase amplifiers. The feedback via the amplifier becomes a positive feedback and causes unstable operation of the circuit and eventually oscillation. If the capacitance of the bypass capacitor 51 (Cs1) is made sufficiently large, the stability will be improved. However, it is disadvantageous in terms of area for monolithic integration. Although the stability is improved, the characteristics in the frequency band to be amplified may be deteriorated due to the parasitic inductance of the capacitor. As a more specific example, transistors 26 and 32
And replaced with the equivalent circuit shown in FIG.
Rb5 = 5 kΩ, Rb4 = Rb6 = 20 kΩ, Cf1 =
20 pF, Rf1 = 1.5 kΩ, Rc1 = Rc2 = 50
When the gain and the stability index (commonly known as K value) of the circuit of FIG. 5 are calculated with Ω, Cc1 = Cs1 = 10 pF, the result is as shown in FIG. The pass band is around 1 GHz. In order for the circuit to operate stably, the K value needs to be 1 or more at all frequencies, but the K value is 1 or less around 20 MHz, which does not satisfy the stability condition.

The present invention has been made in view of the above problems, and has as its object to provide a novel resistor negative feedback transistor amplifier that eliminates the above conventional problems.

[0009]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a bipolar transistor in which a collector bias is applied to a collector terminal, a base bias is applied to a base terminal of the bipolar transistor, and a collector bias is applied to the collector terminal. One terminal of the first capacitor is connected, and a feedback circuit in which a second capacitor and a resistor are connected in series is connected immediately after the other terminal of the first capacitor and the base terminal. Is output from the other terminal of the first capacitor. Further, the frequency characteristic of the gain of the amplifier is made variable by adjusting the capacitance value of the first capacitor.

[0010]

According to the present invention, feedback is applied from the other terminal of the capacitor connected to the collector of the transistor to the base via the resistor. Therefore, the amount of feedback decreases. In other words, the capacitor cancels a part of the gain obtained by the transistor. The complex impedance Z of the capacitor is Z = -j / 2πfC
(F: frequency, C: capacitance of the capacitor) Expression (1): Since the voltage drop in the capacitor is larger as the frequency is lower, the effect of suppressing the gain of the transistor on the low frequency side is different from that of the conventional example. As a result, the flatness of the gain is improved as compared with the conventional example. Further, when a plurality of stages of amplifiers are connected as in the above-described conventional example, even if a positive feedback loop occurs via a bias line, the gain of the transistor at a low frequency is suppressed. The gain decreases and the stability improves. Also, in equation (1), for C satisfying | Z | >> 1, the signal attenuation in the capacitor decreases by 6 dB / octave as the frequency increases, and | Z | << 1 For C, the signal attenuation is 0 dB / octave.

Therefore, the variable range of the gain gradient which can be realized by adjusting the capacitance value of the capacitor is from 0 dB / octave to 6 dB / octave. Normally, the gain of the transistor is reduced at 6 dB / octave. However, the gain gradient is flattened to some extent even in the negative feedback circuit portion. Is possible. In addition,
If the capacitance is selected so that the impedance of the capacitor is sufficiently smaller than the feedback resistance, it is possible to provide an amplifier in which the gain is reduced on the high frequency side as in the conventional example.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention in the case of a one-stage resistive negative feedback transistor amplifier. Here, 1 is an amplifying transistor and 2 is a negative feedback resistor (R
f), 3, 52 and 57 are capacitors (Cf, Cc1, C
s1), 4, 4a are base bias resistors (Rb1, Rb
2), 5 is a collector bias resistance (Rc), Vbb is a base bias, and Vcc is a collector bias. A signal is input from the base terminal 6 and output from the terminal 7.
In this embodiment, resistive feedback from the collector to the base of the transistor is applied via the capacitor Cc1 connected to the collector of the transistor, and the capacitor Cc1
A feature of the present invention is that a signal is output from one end of the present invention. By adopting this circuit configuration, as described in the section of the operation, if the capacitance of the capacitor is reduced, the gain increases on the high frequency side, and if the capacitance of the capacitor is sufficiently increased, the gain decreases on the high frequency side. If the value is intermediate, the frequency characteristic of the gain of the amplifier can be flattened.

Next, another embodiment of the present invention in which an amplifying transistor is connected to a stage subsequent to the amplifier will be described.

FIG. 2 shows a case where an amplifying transistor is connected to the subsequent stage of the amplifier shown in the above embodiment. Here, 8 transistors, 9 negative feedback resistors (Rf1), 53 DC separation capacitors (Cf1), 1
1, 11a base bias resistors (Rb3.Rb4),
The first stage negative feedback amplifier is constituted by the collector bias resistor (Rc1) of 12 and the bypass capacitor (Cs1) of 55, and 56,
56a base bias resistors (Rb5, Rb6), 16
Is connected to the collector bias resistor (Rc2).
The signal is input from the terminal 17 and output from the terminal 18. In the present embodiment, as described in the section of the operation, the gain can be reduced on the low frequency side of the positive feedback loop via the bias line by the capacitor Cc1, so that the circuit can operate stably.

As a more specific example, the transistor 8,
13 is replaced by the equivalent circuit shown in FIG. 6 similarly to the conventional example, and other circuit elements are also Rb3 = Rb5 = 5 kΩ, Rb4
= Rb6 = 20 kΩ, Cf1 = 20 pF, Rf1 = 1.
5 kΩ, Rc1 = Rc2 = 50Ω, Cc1 = Cs1 = 1
When the gain and stability index (commonly referred to as K value) of the circuit of FIG. 2 are calculated in the same manner as the conventional example with 0 pF, the result is as shown in FIG.
Compared with FIG. 7 showing the characteristics of the conventional example, the characteristics in the pass band around 1 GHz are almost the same,
Even in the vicinity of 20 MHz where the value <1, the present example has a K value> 1, which indicates that the circuit is stabilized.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the claims.

[0018]

As described above, according to the present invention, negative feedback that suppresses oscillation caused by unnecessary feedback from a power supply or the like, widens the variable range of gain frequency characteristics, and enables stable operation of a circuit. Type transistor amplifier can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a circuit configuration according to an embodiment of the present invention.

FIG. 2 is a diagram showing a circuit configuration according to another embodiment of the present invention.

FIG. 3 is a diagram illustrating frequency dependence of gain and stability index according to an embodiment of the present invention.

FIG. 4 is a diagram showing a circuit configuration of a conventional transistor amplifier.

FIG. 5 is a diagram showing a circuit configuration of another conventional transistor amplifier.

FIG. 6 is a diagram showing an equivalent circuit of a transistor included in the amplifier.

FIG. 7 is a diagram showing the frequency dependence of the gain and stability index of a conventional transistor amplifier.

[Explanation of symbols]

1, 8, 13, 19, 26, 32 Transistor 2, 9, 20, 27 Negative feedback resistor 3, 10, 21, 28, 31, 50, 51, 52, 5
3,55,57 Capacitors 4,4a, 11,11a, 22,22a, 29,29
a, 35, 35a, 56, 56a Base bias resistor 5, 12, 16, 23, 30, 36 Collector bias resistor 6, 17, 24, 37 Signal input terminal 7, 18, 25, 38 Signal output terminal

Claims (2)

(57) [Claims] A collector bias applied to a collector terminal of the bipolar transistor; a base bias applied to a base terminal of the bipolar transistor; one terminal of a first capacitor connected to the collector terminal; A feedback circuit in which a second capacitor and a resistor are connected in series is connected immediately after the other terminal of the first capacitor and the base terminal, and a signal input to the base terminal is output from the other terminal of the first capacitor. A transistor amplifier. 2. The transistor amplifier according to claim 1, wherein the frequency characteristic of the gain of the amplifier is made variable by adjusting the capacitance value of the first capacitor.