Abstract:
A transistor circuit is formed of at least two unit transistors. Each unit transistor includes an NPN bipolar transistor and a PN junction diode. The P region (anode) of the PN junction diode is connected to the P region (base) of the NPN transistor. The N region (cathode) of the PN junction diode is connected directly or indirectly to the N region (emitter) of the NPN transistor. One of the unit transistors is so connected to the other unit transistor as to form a Darlington circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a transistor circuit, particularly to a transistor amplifier circuit in which changes of its input impedance due to a variation of the current amplification factor of a transistor are effectively prevented. 
     In general, changes of input impedance of a transistor amplifier circuit affect the whole circuit operation. For instance, in a charge/discharge circuit of a sample/hold circuit or in an amplifier circuit for amplifying a signal detected by an AM detector, output level variations due to input impedance changes cause an erroneous or inaccurate operation of the circuit. It is therefore essential to design the circuit so as to reduce the input impedance changes. 
     Such a problem as mentioned above will be described with reference to FIG. 1. In this figure, an amplitude-modulated signal applied to a terminal P1 is envelope-detected by the circuit comprising a base-emitter diode of transistor Q1 and a capacitor C1. The envelope-detected signal is current-amplified by transistors Q2 and Q3 and then outputted from a terminal P2 which is connected to an emitter resistor R1 of the transistor Q3. When the current amplification factors β1 and β2 of the transistors Q2 and Q3 are varied, the output signal level is also varied. This is because the product of the capacitance of capacitor C1, the resistance of resistor R1 and the variations of current amplification factors β1, β2 of transistors Q2, Q3 considerably varies the discharge time constant for the charged capacitor C1. Thus, the variation of current amplification factors of transistors Q2 and Q3 affects the operation of the whole circuitry. In other words, the input impedance of transistor Q2 as well as the discharge time constant are influenced by the product of the current amplification factor variations of transistors Q1, Q2, that is, Δβ1, Δβ2. 
     To make the circuit operate more accurately, input impedance changes of the transistor circuit should be avoided as much as possible. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a transistor circuit which is free from the influence of input impedance changes. 
     To achieve the above object, a transistor circuit of the invention has an improved combination of a current mirror circuit and a Darlington circuit. The current mirror circuit includes a bipolar transistor whose base and emitter are coupled via a PN junction diode. The circuit formed by the transistor and diode constitutes a unit transistor. At least two unit transistors are connected, thus forming a Darlington type (i.e. a Darlington or inverted Darlington) circuit. Due to the operation of the current mirror circuit, the input impedance of the unit transistor is affected very little by a variation of the current amplification factor of the bipolar transistor. The current mirror circuit operation inherently reduces the effective current amplification factor of the unit transistor. But such reduction of the current amplification factor is compensated for by the current boost operation of the Darlington circuit. Accordingly, a high current amplification factor with little variation can be obtained. That is, a transistor circuit which has a sufficiently high and practically constant input impedance can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional transistor circuit whose particular problem is solved by the present invention; 
     FIG. 2 shows an embodiment of the present invention wherein the invention is applied to an AM demodulator; 
     FIG. 3 shows a basic circuit configuration of a unit transistor; 
     FIG. 4 shows a Darlington-connected unit transistor according to the present invention; 
     FIG. 5 shows the PN junction areas of the transistor Q32 and the diode D32 formed in a semi-conductor P-substrate, which indicates that the ratio n of those areas (base-emitter of Q32/junction of D32) is larger than &#34;1&#34;; 
     FIG. 6 shows another embodiment of the unit transistors of the present invention; 
     FIG. 7 shows another embodiment of the unit transistors of the present invention; 
     FIG. 8 shows another embodiment of the unit transistors of the present invention; and 
     FIG. 9 shows the Darlington/inverted Darlington-connected unit transistors according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described with reference to FIGS. 2 to 9, wherein the same or similar reference symbols are used to designate the same or similar portions for brevity&#39;s sake. 
     Referring now to FIG. 2, there is shown an AM demodulator for demodulating an amplitude-modulated signal to which a transistor circuit of the present invention is applied. 
     In FIG. 2, an amplitude-modulated signal is applied to an input terminal P10 and amplified by a preamplifier circuit 10 which comprises transistors Q10-Q13, resistors R10-R16 and an LC resonance circuit including the capacitor C1O and an inductor L1O. The terminal P1O is connected to the base of PNP transistor Q1O and grounded via resistor R1O. The collector of transistor Q10 is grounded. The emitter of transistor Q10 is coupled via resistor R14 to the collector of PNP transistor Q12. The emitter of transistor Q12 is coupled via resistor R13 to power supply VCC. The base of the transistor Q12 is connected to the base and collector of PNP transistor Q11. The emitter of transistor Q11 is coupled via resistor R12 to power supply VCC. The base of transistor Q11 is grounded via resistor R11. The collector of transistor Q12 is connected to the base of NPN transistor Q13. The emitter of transistor Q13 is grounded via resistor R15. The collector of transistor Q13 is coupled via the resistor R16 and the LC resonance circuit to a power supply VCC. 
     The transistor Q10 forms an emitter follower, and the LC resonance circuit provides a means for extracting a given carrier signal with a specific frequency. The modulated signal amplified by preamplifier 10 is supplied to a detector circuit 20 having transistors Q20, Q21 and capacitor C20, and is amplitude-detected. The collector of transistor Q13 is connected to the base of NPN transistor Q20. The collector of transistor Q20 is connected to power supply VCC, and the emitter thereof is connected to the base of NPN transistor Q21. The collector of transistor Q21 is connected to power supply VCC and the emitter thereof is coupled via capacitor C20 to power supply VCC. The detected signal derived from the emitter of transistor Q21 is also amplified by a transistor circuit 30 which includes the three-stage cascade-connected transistors Q30-Q32, resistors R31, R32, R40 and diodes D30-D32. The emitter of transistor Q21 is connected to the base of NPN transistor Q30. The base of transistor Q30 is coupled via the anode-cathode path of PN junction diode D30 to the emitter thereof. The emitter of transistor Q30 is connected to the base of NPN transistor Q31. The base and emitter of transistor Q31 are coupled via the anode-cathode path of PN junction diode D31 and resistor R31. The emitter of transistor Q31 is connected to the base of NPN transistor Q32. The base and emitter of transistor Q32 are coupled via the anode-cathode path of PN junction diode D32 and resistor R32. The emitter of transistor Q32 is connected to an output terminal P20 and grounded via resistor R40. The collectors of respective transistors Q30-Q32 are connected to the power supply VCC. These transistors Q30-Q32 provide a discharge current path for capacitor C20. 
     In FIG. 2, the discharge time constant T is given as: 
     
         T=Zin·C20                                         (1) 
    
     where Zin is the input impedance measured from the base circuit of transistor Q30 and C20 is the capacitance of capacitor C20. The impedance Zin depends on the product of the current amplification factors β30-β32 of transistors Q30-Q32 and the resistance of resistor R40, i.e. Zin α β30·β31·β32·R40. Accordingly, to suppress changes of the input impedance Zin, each current amplification factor of transistors Q30-Q32 should be constant. 
     In this invention the current amplification factor variation of each transistor is effectively lessened by a specific circuit, i.e., a so-called &#34;current mirror circuit&#34;. An equivalent or effective current amplification factor of the current mirror transistor is, however, decreased by the current mirror operation. This drawback is overcome by combining a Darlington connection with plural current mirror transistors. This is the key point of the present invention. The effective current amplification factor of a current mirror transistor may optionally be determined by the ratio of the emitter area of a transistor to the junction area of a diode, the transistor and diode comprising the current mirror circuit. 
     Now, minimization of the current amplification factor variation of the current mirror transistor will be explained in detail. 
     FIG. 3 shows the current mirror circuit of transistor Q32 which hereinafter shall be referred to as a unit transistor Q32&#39;. In FIG. 3, I1 denotes the emitter current of transistor Q32, I2 denotes the current flowing through diode D32 and resistor R32, IB1 denotes the base current of transistor Q32 and IC denotes the collector current of transistor Q32. 
     In FIG. 3, the effective current amplification factor β32&#39; is defined by the ratio of the base current IB of unit transistor Q32&#39; and the collector current IC thereof. That is, assuming β32&gt;&gt;1: ##EQU1## The currents I1 and I2 can be given: 
     
         I1=nA exp{(q/kT)VF1}                                       (3) 
    
     
         I2=A exp{(q/kT)VF2}                                        (4) 
    
     
         I1=(1+β32)IB1                                         (5) 
    
     where k is Boltzmann&#39;s constant, T is the absolute temperature, VF1 is the base-emitter voltage of transistor Q32, VF2 is the PN junction voltage of diode D32, A is the area of PN junction of diode D32, nA is the PN junction area of the emitter of transistor Q32, and n is the ratio of &#34;area of Q32 emitter junction/area of D32 junction&#34;. 
     From equations (2) to (5), β32&#39; is: ##EQU2## Further, 
     
         VF1=VF2+R32·I2                                    (7) 
    
     From equations (6) and (7), β32&#39; is given: ##EQU3## 
     Equation (8) represents the effective current amplification factor β32&#39; of the unit transistor Q32&#39; and indicates that the value of β32&#39; depends chiefly on the second term of the denominator of equation (8), because β32 of bipolar transistor Q32 is much larger than &#34;1&#34;. Accordingly, even though the value of β32 is varied, β32&#39;--the effective current amplification factor of the unit transistor Q32&#39;--is changed very little. That is, Δβ32&#39;/Δβ32 is quite small. This means that the effective current amplification factor β32&#39; of unit transistor Q32&#39; is practically constant. 
     Equation (8) also suggests that β32&#39; smaller than β32. This disadvantage inherent to the unit transistor Q32&#39; can be removed by the Darlington connection of plural unit transistors. FIG. 4 shows one example of Darlington connections of three unit transistors Q30&#39;-Q32&#39;. 
     In FIG. 4, the collector of an NPN double-emitter transistor Q30 is connected to a power supply VCC. The base and emitter of transistor Q30 are coupled via the anode-cathode path of a diode D30. The emitter of transistor Q30 is connected to the base of an NPN double-emitter transistor Q31. The base and emitter of transistor Q31 are coupled via the anode-cathode path of a diode D31 and a resistor R31. The collector of transistor Q31 is connected to power supply VCC and the emitter thereof is connected to the base of an NPN transistor Q32. The base and emitter of transistor Q32 are coupled via the anode-cathode path of a diode D32 and a resistor R32. The collector of transistor Q32 is connected to power supply VCC and the emitter thereof is grounded through a load resistor R40. The transistor circuit 30 shown in FIG. 4 has a high and constant input impedance Zin by means of the effective combination of the three current mirror circuits (unit transistors Q30&#39;-Q32&#39;) with the Darlington connection. 
     Now, consideration will be given to the determination of the number of unit transistors which should be Darlington-connected. Let N denote the number of Darlington-connected unit transistors, β1, β2, β3, . . . , βN denote the respective current amplification factors of unit transistors, and β0 denote the desired current amplification factor. Then, the following should be satisfied: 
     
         β0=β1·β2·β3 . . .    βN (9) 
    
     For example, when β1=β2=β3=10 and β0 is required to be 1,000, then three-stage Darlington circuit (N=3) as shown in FIG. 4 is used. 
     Incidentally, equation (8) may be simplified if the value of β32 is much larger than &#34;1&#34;. That is: 
     
         β32&#39;≃n·exp {(q/kT)·R32·I2}) (10) 
    
     Equation (10 ) teaches that a large n permits a reduction in R32 when the desired value of β32&#39; is fixed. Accordingly, the transistor circuit of the present invention is well suited for IC fabrication. In IC fabrication, since n is determined by the ratio A2/A1 of the diffusion areas A1 and A2 as seen from FIG. 5, it is easy to make n large. However, it is difficult to make a high and accurate resistance (R32) in an IC. 
     In FIG. 2, a unit transistor Q30&#39; including transistor Q30 and diode D30 has the resistance R30=0. Accordingly, the current amplification factor β30&#39; of unit transistor Q30&#39; is β30&#39;=n, as evident from equation (10). Further, if n=2, the current amplification factors of unit transistors Q31&#39; and Q32&#39; are β31&#39;=2exp{(q/kT)·R31·I2} and β32&#39;=2exp{(q/kT)·R32·I2}, respectively. Since the circuit 30 of FIG. 2 has a constant input impedance, the discharge time constant for capacitor C20 remains unchanged. Therefore, an erroneous or inaccurate circuit operation due to input impedance changes is avoided. 
     FIG. 6 shows another equivalent circuit of the unit transistor. A diode-connected transistor D32A is used for the PN junction diode of FIG. 3 and a transistor Q32 has the emitter connected to a resistor R33. The resistor R33 may be used for adjusting the effective current amplification factor of the unit transistor (Q32, D32A, R32 and R33). The resistances of resistors R32 and R33 may of course be zero. 
     FIG. 7 shows still another equivalent circuit of a unit transistor. Bipolar transistors Q32A and Q32B are Darlington-connected for increasing the current amplification factor of transistor Q32B. In this case, a diode D32 may have two series-connected PN junctions. 
     FIG. 8 shows another equivalent circuit of a unit transistor. Bipolar transistors Q32A and Q32C are inverted Darlington-connected, and a diode-connected transistor D32B is used. 
     FIG. 9 shows a combination circuit of Darlington/inverted Darlington-connected unit transistors. The circuit may be used for the transistor circuit 30 of FIG. 2. In FIG. 9, a unit transistor Q30&#39; (NPN) and a unit transistor Q33&#39; (PNP) form an inverted Darlington circuit, and a unit transistor Q32&#39; (NPN) as well as the transistors Q30&#39; and Q33&#39; form a Darlington circuit. 
     As apparent from the above, since the transistor circuit of the invention comprises Darlington-connected unit transistors each including a bipolar transistor and a diode coupled between the base-emitter of the transistor as shown in FIG. 4, it is free from the influence of the current amplification factor variation of a bipolar transistor. 
     It is noted that the present invention may be applied not only to a sample/hold circuit or an AM circuit but also to any other proper circuit.