Limiter Amplifier

The present invention relates to a limiter amplifier suitable for use in a radio communication unit, more particularly in a portable communication unit, and such as a paging receiver.

Compactness and light weight are strongly required on this kind of portable communication unit, which uses a limiter amplifier in addition to a frequency discriminator, to demodulate an FM carrier wave.

One example of conventional limiter amplifiers includes a plurality of amplifiers of the grounded-emitter type, connected in cascade by means of coupling capacitors. The amplifier has upper and lower limits which are determined by a saturated collector voltage, i.e., a power source voltage. Therefore, the use of a low-voltage power source, such as a single dry battery, may provide a distorted waveform having asymmetric upper and lower limits at the output of the amplifier.

In addition, such a limiter amplifier cannot be fabricated in a monolithic integrated circuit because coupling capacitors are required. Moreover, since conventional limiter amplifiers have no temperature protection, they involve another problem because their saturation characteristics vary with changes in ambient temperature.

An object of the present invention, therefore, is to provide a limiter amplifier which is capable of operating on a low-voltage power source, such as a single dry battery, and having a favorable saturation characteristic.

Another object of the present invention is to provide a limiter amplifier having a favorable saturation characteristic by keeping the current constant even if there are changes in ambient temperature.

Still another object of the present invention is to provide a limiter amplifier well adaptive to construction in the form of a monolithic integrated circuit.

According to the invention, a limiter amplifier comprises a power supply means. A plurality of cascade-connected differential amplifiers are provided for amplitude-limiting an input signal. Each of the amplifiers has a constant current circuit with a biassing circuit connected to the input of the differential amplifier for supplying it with a biassing voltage. The biassing circuit has a second constant current circuit. A third constant current circuit activates the first and second constant current circuits. A voltage regulator is connected to the power supply means for supplying a constant voltage to the first to third constant current circuits. The temperature related characteristics of the voltage regulator are the same as the temperature related characteristics of the first to third constant current circuits.

Other objects, features and advantages of the present invention will become more apparent from the detailed description hereunder taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic circuit diagram of a conventional limiter amplifier;

FIGS. 2A and 2B respectively show the current-voltage characteristic of each transistor shown in FIG. 1 and the limited output waveform of the circuit of FIG. 1;

FIG. 3 is a schematic circuit diagram of the limiter amplifier according to the present invention;

FIGS. 4A and 4B respectively show the current-voltage characteristic of each transistor shown in FIG. 3 and the limited output waveform of the circuit of FIG. 3;

FIG. 5 shows the thermal characteristics of the power source voltage, and the D.C. operating points and output voltages of the transistors in the limiter amplifier shown in FIG. 3; and

FIG. 6 is a schematic circuit diagram of the voltage regulator for use in the limiter amplifier shown in FIG. 3.

Referring to FIG. 1, four grounded-emitter amplifiers respectively have transistors 6 to 9, collector resistors 16 to 19 and feedback bias resistors 21 to 24. These amplifiers are coupled to one another by capacitors 11 to 14, respectively. Reference numerals 1, 2, 3 and 4 respectively represent an input terminal, an output terminal, a power supply terminal and a resistor for determining the input impedance.

It is assumed that the voltage at the power supply terminal 3 is represented by V.sub.CC1, the base-emitter voltage of the transistors 6, 7, 8 and 9 by V.sub.BE, the D.C. amplification factor of the transistors by h.sub.FE, the resistance of the collector resistors 16, 17, 18 and 19 by R.sub.C1 and the resistances of the feedback bias resistors 21, 22, 23 and 24 by R.sub.B1. As will be seen from FIG. 2A, the D.C. operating points I.sub.CO1 and V.sub.CO1 of the transistors 6, 7, 8 and 9 can be defined by the following equations, respectively: ##EQU1##

When an input signal of large amplitude is applied to terminal 1, the output voltage of the transistor 6, 7, 8 or 9 ranges between its saturated collector voltage V.sub.CE(sat) and the power source voltage V.sub.CC1. The output of the limiter amplifier will have a favorable saturated waveform with a 50 percent duty factor if the following equation is satisfied: ##EQU2##

Supposing that V.sub.CE(sat) =0.15 (V), V.sub.BE =0.65 (V), I.sub.CO1 =40(.mu.A), R.sub.B1 =180(K.OMEGA.) and h.sub.FE =100, ##EQU3## The power source voltage V.sub.CC1 then required will be

However, the supply of 1.29 (V) from a single dry battery would extremely shorten the useful life of the battery, and a nickel cadmium battery would be unusable in this case. If a dry battery is used, its terminal voltage is usually somewhere between 1.0 and 1.05(V). Accordingly, Equation (3) above is not satisfied, resulting in the disadvantage that the output waveform A of the prior art limiter amplifier shown in FIG. 1 is distorted when saturated, as shown in FIG. 2B.

There is another disadvantage because the presence of the coupling capacitors 11, 12, 13 and 14 between the transistors 6, 7, 8 and 9 makes the conventional limiter amplifier unsuitable for production as a monolithic integrated circuit.

Referring now to FIG. 3, a limiter amplifier according to the present invention includes a voltage regulator 26, a circuit in which differential amplifiers 27 and 28 are directly coupled to each other, a biassing circuit 29, and a feedback circuit 30. The voltage regulator 26, deriving its input from a dry battery 32, has a temperature declination such that the currents through transistors 33, 34, 35 and 36 remain constant irrespective of temperature variations. Voltage regulator 26 supplies a regulated voltage to these transistors through a power supply terminal 3. The transistor 36 provides the transistors 33 to 35 with a constant bias voltage. The differential amplifier 27, driven by the transistor 34, has resistors 38 and 39 and transistors 40 and 41. Similarly, the differential amplifier 28, driven by the transistor 35, has resistors 43 and 44 and transistors 45 and 46. The biassing circuit 29 includes the transistor 33 and a resistor 48, and the feedback circuit 30 comprises a capacitor 50 and a resistor 49 for effecting a D.C. feedback from the collector electrode of the transistor 45 to the base electrode of the transistor 41. Together with the transistor 36, the transistors 33 to 35 constitute constant current circuits to supply constant currents to the biasing circuit 29 and the differential amplifiers 27 and 28, respectively.

In such a circuit structure, the collector current I.sub.O of the transistor 36 is obtained by the following approximation: ##EQU4## where V.sub.CC2 is the voltage at the power supply terminal 3, R.sub.51 is the resistance of a resistor 51 and V.sub.BE is the base-emitter voltage of the transistor 36. If the transistor 36 has the same characteristic as the transistors 33, 34 and 35, the collector currents of each of these transistors 33, 34 and 35 will be equal to I.sub.O. Meanwhile, if the resistances of the resistors 38, 39, 43 and 44 are all R.sub.C2, the collector currents of the transistors 40, 41, 45 and 46 will be I.sub.O /2, and the operating points I.sub.CO2 and V.sub.CO2 of the differential amplifiers 27 and 28 are: ##EQU5##

FIG. 4A shows the collector current vs. collector voltage characteristic of the transistor 40 constituting the differential amplifier 27. In FIG. 4A, the maximum collector current is determined by the constant current I.sub.O, beyond which no current will flow through the collector electrode. Therefore, the differential amplifier 27 acts as a current mode limiter. When a large signal is supplied to an input terminal 1, the collector output waveform of the transistor 40 will be as shown at A' in FIG. 4B, which is a highly favorable saturated waveform. Thus, a limiter amplifier operates with a low-voltage power source and yet has a favorable saturation characteristic, even with directly coupled differential amplifiers 27 and 28, whose operating points as to satisfy Equations (5). The direct coupling of the differential amplifiers 27 and 28 further provides a limiter amplifier which is suitable for monolithic circuit integration.

The temperature variation rate of Equation (4) is represented by: ##EQU6## If the condition of ##EQU7## is satisfied, the following equation is given: ##EQU8## Thus the constant current I.sub.O is insensitive to temperature variations. Then, from Equations (5), the following equations will hold with respect to the operating points of the differential amplifiers 27 and 28: ##EQU9## This collector operating point V.sub.CO2 will have the same temperature declination as the voltage V.sub.CC2 at the power supply terminal 3, i.e. that of the base emitter voltage V.sub.BE of the transistor 36, The thermal variance is shown in FIG. 5. In FIG. 5, lines B represent the collector output range of the transistor 40, 41, 45 or 46; C represents the base-emitter voltage range of transistor 40; and line D represents, the collector-emitter voltage range of transistor in the constant current circuit.

The transistor 40, under the above stated conditions, supplies large signals ranging from (2V.sub.CO2 -V.sub.CC2) to V.sub.CC2. The value of

does not vary with temperature. Thus, the saturated waveform will remain highly favorable. Moreover the value of 2(V.sub.CC2 -V.sub.CO2) will not vary, even if the temperature changes.

Further, the resistance of the resistor 48, in the base biassing circuit 29 for the transistor 40 is represented by R.sub.48. The base voltage V.sub.B of the transistor 40 is

because the collector current of the transistor 33 is the constant current I.sub.O. The variation with respect to temperature is ##EQU10## equal to that of the regulated voltage at the power supply terminal 3. The resistance R.sub.48 of the resistor 48 can be as selected as to make a node voltage a equal to node voltages b and c. In this particular embodiment of the invention, the resistance R.sub.48 is ##EQU11## and node voltages a, b, c, d, e and f are all equal to each other. Thus, the waveform delivered to the output terminal 2 of the limiter amplifier will have as favorable a saturated form as the collector output waveform of the transistor 40.

Therefore, the output waveform at the output terminal 2 will be from (2V.sub.CO2 -V.sub.CC2) to V.sub.CC2. The saturated waveform will remain highly favorable even if the temperature varies, with the value of 2(V.sub.CC2 -V.sub.CO2) unchanged. Thus, the voltage regulator 26, which supplies a regulated voltage having a temperature declination satisfying the condition of ##EQU12## makes all of the node voltages a, b, c, d, e and f equal. The differential amplifiers 27 and 28 are directly coupled to each other and to the biassing circuit 29. The limiter amplifier has a highly favorable saturation characteristic, against the voltage fluctuation of the dry battery 32 and against temperature variations.

The bias voltage is stabilized by feeding a D.C. voltage back to the base electrode of the transistor 41, from the collector electrode of the transistor 45, and through a low-pass filter comprising the resistor 49 and the capacitor 50. The differential amplifier 27, comprising the transistors 40, 41 and 34, detects any error in the voltage, and operates to equalize the base voltage f of the transistor 41 with the base voltage a of the transistor 40. The node voltages a and f are thereby made equal. Since the resistors 38, 39, 43 and 44 have the same resistances, the node voltages a to f are equal to one another. The node voltages a to f have to be greater than the sum of the base-emitter voltage V.sub.BE and the collector-emitter voltage V.sub.CE of the transistor 34. Now supposing that the base-emitter voltage V.sub.BE of the transistor 40 is 0.65 (V) and the collector-emitter voltage V.sub.CE of the transistor 34 is 0.2 (V), the limiter amplifier will be fully operable at node voltages a to f of 0.85 (V) and a regulated output voltage of 1.05 (V).

FIG. 6 illustrates in detail the structure of the voltage regulator 26, referred to in FIG. 3. In FIG. 6, the voltage regulator 26 comprises a band-gap reference voltage generating section 53, a voltage detecting section 54, an error amplifying section 55, a voltage regulating section 56 and a power source decoupling capacitor 57. The band-gap reference voltage generating section 53 has a constant current circuit 58, a resistor 59, a transistor 60, resistors 61 and 62, and transistors 63 and 64. The operation of this circuit is described in detail in a paper entitled "New Developments in IC Voltage Regulators", Robert J. Widlar, IEEE Journal of Solid-State Circuits, Vol. SC-6, pp. 2-7, Feb. 1971. The voltage detection section 54 has resistors 65 and 66; the error amplifying section 55 has transistors 68, 69, 70 and 71. The constant current circuit 72; and the voltage regulating section 56 has transistors 74 and 75.

As described in the Widlar paper, the reference voltage V.sub.ref at point P in FIG. 6, is: ##EQU13## and its differentation with respect to temperature is given by: Where: V.sub.g0 is the extrapolated energy band-gap voltage for the semiconductor material at absolute zero; q is the charge of an electron; k of Boltzman's constant; T is absolute temperature; V.sub.BEO is the base-emitter voltage at T.sub.0 and I.sub.CO ; and J is the current density. The resistances of the resistors 65 and 66 is represented by R.sub.65 and R.sub.66, respectively. The output voltage V.sub.CC2 at the power supply terminal 3 of the voltage regulator 26 is given by: ##EQU14## and the temperature characteristic of the output voltage V.sub.CC2 is:

Here, the simultaneous Equations (8) to (11) are solved to set the output voltage V.sub.CC2 at a prescribed value and to satisfy the condition of ##EQU15## In this embodiment, V.sub.CC2 is set at 1.05(V) and .iota.V.sub.CC /T at 1.8(mV/.degree.C.). The resistance R.sub.59, R.sub.61, R.sub.62, R.sub.65 and R.sub.66 of the resistors 59, 61, 62, 65 and 66, respectively, are R.sub.59 =4(K.OMEGA.), R.sub.61 =14(K.OMEGA.), R.sub.62 =2.5(K.OMEGA.), R.sub.65 =28.4(K.OMEGA.), and R.sub.66 =76.6(K.OMEGA.).

As described above, according to the present invention, all of the circuit constituents including the voltage regulator, the differential two-stage cascade-connected amplifiers and the biasing circuit are directly coupled and, therefore, are suitable for monolithic integration into a single semi-conductor chip. Also, it is possible for the present invention to produce a device with well-matched characteristics by monolithically integrating the constituent transistors and resistors and by arranging them on a single chip. The resulting device, which is so produced, has the advantage of manifesting a favorable saturation characteristic against voltage fluctuations and temperature variations.

The limiter amplifier according to the invention has another advantage of being less susceptible to fluctuation in current consumption, owing to production errors, or the like, and of being subject to no variation in current consumption with a change in temperature, resulting in a reduction of battery consumption.

Furthermore, the voltage regulator can also supply power of stable voltage to other devices within the unit, such as a pager receiver.