Transistor direct-coupled amplifier

Abstract: A transistor direct-coupled amplifier includes a positive-phase direct-coupled amplifier circuit (1), a second transistor (Q1), and a low-pass filter (LDF). The positive-phase direct-coupled amplifier circuit uses a base of a first transistor (Q2) as an input terminal. The second transistor Q1 has a collector connected to the input terminal of the positive-phase direct-coupled amplifier circuit (1) and an emitter connected to a reference voltage source (2), and the second transistor is complementary with the first transistor. A low-pass filter (LDF) receives an output from the positive-phase direct-coupled amplifier circuit as an input, and the low-pass filter is connected to feed back an output to a base of the second transistor.

Description: The present invention relates to an amplifier and, more particularly, to a transistor direct-coupled amplifier for amplifying a DC balanced signal. In general, although many types of monolithic ICs called wide-range amplifiers are used, when these ICs are to be applied as wide-range amplifiers without being matched with their characteristics, they may not be used as the wide-range amplifiers due to no degree of freedom of design. In contrast to this, in an amplifier constituted by a combination of transistor circuits, circuit design matched with an application purpose can be obtained. Therefore, the amplifier is frequently used in a video circuit and the like due to its high degree of freedom of design. In a conventional direct-coupled amplifier, in order to effectively use the dynamic range of an output voltage to reach a power source voltage, when an operating point is set to be about 1/2 the power source voltage, variable resistors are used as some resistors of a bias circuit and adjusted to absorb variations in transistors and resistors. In such a direct-coupled amplifier, when the operating point is set to be about 1/2 the power source voltage, some of resistors of the bias circuit must be changed into variable resistors VR11 to absorb variations in transistors and resistors, thereby posing some problems in the manufacture of the amplifier. In addition, after the variable resistors are adjusted, the amplifier has no countermeasure against a change in element value with time, a change in power source voltage, and a change in temperature. Especially, since a circuit having a high voltage gain has a small input signal level, the circuit is sensitive to a change in DC operating point caused by these changes, a signal clamping distortion is disadvantageously generated caused by a shift in the operating point. In order to prevent this, an output level must be set to be low, or a power source voltage must be increased. These limitations are drawbacks in use of the circuit. In the specification of United States patent number 3,452,281, which was published on June 24 1969, there was proposed a circuit for providing temperature compensation for a transistor by means of a diode connected in a biassing circuit for the input electrode of the transistor. Proper current for the diode was derived from a resistor network. In the specification of United States patent number 3,899,742, which was published on August 12 1975, there was proposed an apparatus and method for operating a driver element of an amplifier at a low DC output current in response to a low level fluctuating input signal, and for increasing the DC output current in response to an increase in the magnitude of the fluctuating input signal level, so that the input signal was linearly amplified throughout a wide range of amplitudes. Features of a transistor direct-coupled amplifier to be described below are that it is capable of optimally setting an output operating point to an appropriate voltage at any time, that no initial adjustment by a variable resistor is required, and that it can be made free from a change in element value with time, a change in power source voltage, and a change in temperature. In a particular arrangement to be described below the base of one transistor of a positive-phase direct-coupled amplifier circuit is used as an input terminal, the collector of another transistor is connected to the input terminal, and the emitter of the another transistor is connected to a reference voltage source, the another transistor being complementary to the one transistor, and a low-pass filter is connected to feed back an output from the output of the positive-phase direct-coupled amplifier circuit to the base of the one transistor. The following description and drawings disclose, by means of an example, the invention which is characterised in the appended claims whose terms determine the extent of the protection conferred hereby, together with the circuit of a conventional amplifier. In the drawings:- Fig. 1 is a circuit diagram of an amplifier which illustrates an embodiment of the present invention; Fig. 2 is a circuit diagram showing a low-pass filter suitable for use in the circuit of Fig. 1, and Fig. 3 is a circuit diagram showing a conventional amplifier. Description of the Preferred Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a circuit diagram showing an amplifier according to the embodiment of the present invention, and Fig. 2 is a circuit diagram showing a low-pass filter LPF having a resistor R0 and a capacitor C0 in Fig. 1. A positive-phase direct-coupled amplifier circuit 1 is constituted by an npn transistor Q2 and a pnp transistor Q3. That is, a collector output from the npn transistor Q2 is input to the base of the pnp transistor Q3, and the collector of the pnp transistor Q3 is connected to the emitter of the npn transistor Q2 through a resistor R3 and is negatively fed back. Assuming that R1 = R3 = kR2 = kR4 (k is a constant >> 1, R1 to R4: resistances of the resistors R1 to R4, respectively), the gain of this circuit is about k times. A reference voltage source circuit 2 is constituted by a series circuit. The series circuit is obtained such that two resistors R5 and R6 each having the same resistance and diodes D1 and D2 are arranged between a positive power source terminal V CC (voltage: V CC ) and ground. When the resistances of the resistors R5 and R6 are set to be low such that a change in voltage caused by an emitter current of the transistor Q1 can be neglected, a midpoint voltage of the diodes D1 and D2 goes to V CC /2, and the emitter voltage of the pnp transistor Q1 goes to V CC /2 + V D , where V D represents an ON voltage (about 0.6 V) of the diode D1. An operation of the circuit will be described below. In the amplifier shown in Fig. 1, the pnp transistor Q1 is normally set in an ON state for the following reason. That is, if the pnp transistor Q1 is set in an OFF state, the npn transistor Q2 has no base current and is set in an OFF state, and the pnp transistor Q3 has no base current and is set in an OFF state. Therefore, currents from the two transistors Q2 and Q3 are not supplied to the resistors R2 and R3, respectively, the voltage of an output terminal OUT goes to 0 V, and an output voltage of 0 V appears at an output terminal F OUT of the low-pass filter LPF. At this time, a current path is formed by the power source V CC , the resistor R5, the emitter-base path of the pnp transistor Q1, the output terminal F OUT of the low-pass filter LPF, a resistor R0 (Fig. 2) in the low-pass filter LPF, the input terminal F IN of the low-pass filter LPF, the resistor R3, the resistor R2, and ground in this order, and the pnp transistor Q1 is turned on. That is, the pnp transistor Q1 is not turned off but is normally set in an ON state. An input signal to this amplifier is supplied to an input terminal IN through an input capacitor (not shown) not to influence an operating point of an input stage. A DC balanced signal (DC component = 0) is used as the input signal, a voltage equal to a DC voltage at the operating point of the circuit output terminal is generated at the output of the low-pass filter LPF having a DC gain of 1 by removing a signal component. This voltage becomes the base voltage of the pnp transistor Q1, the value of the voltage is smaller than that of the emitter voltage by a base-emitter voltage V BE (about 0.6 V) of the pnp transistor Q1, and the value V CC /2 + V D - V BE ) goes to about V CC /2 by cancelling the ON voltage V D of the diode D1 and the base-emitter voltage V BE of the pnp transistor Q1: That is, the operating point of the voltage at the output terminal OUT of the amplifier is V CC /2. Therefore, the operating point is proportional to the value V CC in a change in power source voltage, and the operating point is always set to be V CC /2. In case of a change in temperature, since the ON voltage of the diode D1 and the base-emitter voltage V BE of the pnp transistor Q1 have the same temperature coefficient, the changes in these voltages cancel each other. The operating point voltage of the circuit output terminal OUT is set to be V CC /2. When a power source is charged, if the operating point of the voltage at the circuit output terminal OUT is higher than V CC /2, the output voltage of the low-pass filter LPF is higher than V CC /2, thereby turning off the pnp transistor Q1. At this time, as described above, the voltage of the output terminal OUT of this amplifier is decreased to be stabilized at a point equal to V CC /2. Since the input impedance of the low-pass filter LPF is set to be higher than the output impedance of the positive-phase direct-coupled amplifier circuit not to influence a degree of voltage amplification, a condition, i.e., "the resistance of the resistor R0 >> the resistance of the resistor R3", is selected. Fig. 3 is a circuit diagram showing a conventional direct-coupled amplifier. In Fig. 3, a pnp transistor Q12 is used in the second stage of the amplifier to increase utilization efficiency of the power source voltage, and the amplifier has a circuit arrangement suitable for requiring a large output amplitude. As described above, since optimal bias conditions are different in accordance with transistors used in the amplifier, a variable resistor VR11 is arranged in series with a resistor R11 to be adjusted. As described above, an output from the positive-phase direct-coupled amplifier circuit is fed back to an input stage through a low-pass filter to perform DC feedback. Therefore, an output operating point can be automatically set to be an optimal voltage (in many cases, about 1/2 the power source voltage) at any time. Initial adjustment performed by a variable resistor is not required, the output operating point can always be kept at an optimal setting point which is designated in advance in case of a change in power source voltage, a change in temperature, and a change in element value with time. It will be understood that, although the invention has been illustrated by means of a particular example, variations and modifications thereof, as well as other embodiments may be made within the scope of the appended claims.

Claims: A transistor direct-coupled amplifier including first and second direct coupled transistors (Q2, Q3), an input (IN), the input (IN) being connected to the base of the first transistor (Q2), and an output (OUT), the collector of the second transistor (Q3) being connected directly to the output (OUT), characterised in that there is provided a third transistor (Q1) having its collector connected directly to the input terminal (IN) and its emitter connected to a reference voltage source (2), in that the first transistor (Q2) and the third transistor (Q1) are complementary, in that the first and the second transistors (Q2) (Q3) provide a positive-phase direct-coupled amplifier circuit (1) and in that a low pass filter (LPF) is connected between the output (OUT) and the base of the third transistor (Q1) to provide feed back from the output (OUT) to the base of the third transistor (Q1) and to set the operating point of the voltage at the output (OUT). An amplifier as claimed in claim 1, wherein the reference voltage source (2) is constituted by a series circuit of first and second resistors (R5, R6) connected between a power source and ground and a series circuit of first and second diodes (D1, D2) connected in series between the first and second resistors (R5, R6), wherein the emitter of the third transistor (Q1) is connected between the first resistor (R5) and the first diode (D1), and an ON voltage (V D ) of the first diode (D1) and the base-emitter voltage (V BE ) of the third transistor (Q1) are cancelled to provide as an operating point of the output voltage of the positive-phase direct-coupled amplifier circuit (1), a divided voltage obtained by dividing a power source voltage (V CC ) between the first and second resistors (R5, R6). An amplifier as claimed in claim 2, wherein the first and second resistors (R5, R6) have the same resistance, and the operating point of the output voltage of the positive-phase amplifier circuit (1) is set to be 1/2 the power source voltage (V CC ). An amplifier as claimed in claim 2, wherein the ON voltage of the first diode (D1) and the base-emitter voltage of the third transistor (Q1) have the same temperature coefficient. An amplifier as claimed in claim 1, wherein the first transistor (Q2) is an npn type and the second transistor (Q3) is a pnp type having its base connected to the collector of the first transistor (Q2), and wherein the collector of the second transistor (Q3) is connected to the base of the third transistor (Q1) which is a pnp type through the low-pass filter (LPF).

Inventors: OHNO MASAHIKO, OHNO, MASAHIKO, Ohno, Masahiko, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H03F3
IPC codes: H03F3, H03F1