Differential amplifier

Abstract: A differential amplifier (Fig. 1) includes switching FETs provided (1, 2) at the input side and at the reference side, respectively, forming a differential pair, an FET serving as a constant current source (6) for flowing a current of a sum of the currents flowing through the respective Switching FETs (1, 2), and an inductor (7) connected in series to the FET serving as a constant current source (6) between the connection node of the both switching FETs (1, 2) and the power supply of the circuit. Therefore, the inductor (7) does not become a load with respect to DC, a fluctuation of the signal current flowing through the FET serving as a constant current source (6) is made small, whereby a difference in the output voltage is reduced.

Description: FIELD OF THE INVENTION The present invention relates to a differential amplifier and, more particularly, to a construction of the differential amplifier that outputs two phase signals having equal amplitudes in response to a single phase input signal. BACKGROUND OF THE INVENTION Figur 7 is a diagram illustrating a prior art differential amplifier having a single phase input and two phase output. In the figure, reference numeral 1 designates a field effect transistor (hereinafter referred to as FET) Q1 having its gate as an input terminal. Reference character IN designates an input terminal to which a signal to be amplified is input, and this input terminal is connected to a gate of the FET Q1. Reference numeral 2 designates an FET Q2 whose gate terminal is connected to a minus side terminal of the reference voltage source V REF and a source terminal thereof is commonly connected to the source terminal of the FET Q1. A load resistance R1 is connected to a drain terminal of the FET Q1. A load resistance R2 is connected to a drain terminal of the FET Q2. An anode electrode of a level shift diode D1 is connected to the ground and a cathode electrode of the level shift diode D1 is connected to the other ends of the load resistances R1 and R2. A constant current source FET Q3 has its gate terminal and source terminal both connected to the power supply V SS and a drain terminal connected to the connection node of the source electrodes of the FET Q1 and FET Q2. Reference character V REF designates a reference voltage source whose plus side terminal is connected to the ground and the minus side terminal of the reference voltage source V REF is connected to the gate terminal of the FET Q2. Reference character OUT designates an output terminal connected to the drain terminal of the FET Q2. Reference character /OUT designates an output terminal connected to the drain terminal of the FET Q1. An amplified signal of the same phase as that of the signal input to the input terminal IN is output from the output terminal OUT and an amplified signal of the reverse phase to that of the signal input to the input terminal IN is output from the output terminal /OUT. A differential amplifier is constructed by the above described circuit elements FET Q1, FET Q2, R1, R2, D1 and FET Q3. Figures 8(a)-8(d) show timing waveforms of the voltage applied to respective terminals and the current flowing through respective FETs in the prior art differential amplifier shown in figure 13. Figure 8(a) is a diagram for explaining the circuit parameters of the prior art differential amplifier shown in figure 13 which are established so as to obtain the timing waveforms of figures 8(b)-8(d). In the graph shown in figure 8(b), broken line shows the input voltage V IN input to the input terminal IN, one point chain line Shows the value of the power supply V REF , and a real line shows the drain terminal voltage V D3 of the FET Q3. The input signal is -3.6 ± 0.5 V, the reference voltage source V REF is -3.6 V, the power supply V SS is -5.2 V, and the period is 500ps(=0.5ns). The second stage graph shown in figure 8(c) shows the two phase output waveforms, and the broken line shows the voltage waveform V /OUT of the reverse phase output /OUT, and the real line shows the waveform V OUT of the positive phase output OUT. The bottom graph shown in figure 8(d) shows current waveforms of the drain currents I1, 12 and 13 of the respective FETs Q1, Q2 and Q3, and the real line shows a current waveform of I1, a broken line shows a current waveform of 12, and one point chain line shows a current waveform of 13. A description is given of the operation. When the input voltage input to the terminal IN is at low level (-3.6 - 0.5 V), the gate source terminal voltage V GS1 of the FET Q1 becomes V GS1 ≒ V TH (=0.1 V) for the threshold voltage V TH and the FET Q1 is turned off. On the other hand, since the FET Q2 has a gate source terminal voltage of V GS2 ≒ 1V (>>V TH ), the FET Q2 is turned on, whereby a current of I2 ≒ I3 (the difference between I2 and I3 in the figure is a gate source current I GS2 of the FET Q2) flows therethrough. Next, when the input voltage is at high level (-3.6 + 0.5 V), V GS1 ≒ 1V and the FET Q1 is turned on, whereby a current of I1 ≒ I3 (the difference between I1 and I3 in the figure is a gate source current I GS2 of the FET Q2) flows therethrough, thereby turning off the FET Q2. Thus when the FET Q2 is turned off, it should be V GS2 = V TH . However, since the gate terminal of the FET Q2 is fixed to a constant voltage (V REF = -3.6V), the drain terminal voltage V D3 of the FET Q3 serving as a constant current source is raised, thereby turning off the FET Q2. Accordingly, the drain terminal voltage of the FET Q3 changes with a constant amplitude of ΔV D3 ≒ (input voltage amplitude )/2 - V TH . The current flowing through the FET Q3 serving as a constant current source changes (by ΔI3) in proportional to the change ΔV D3 of the drain terminal voltage. Since ΔV D3 = ΔI3 x R DS3 (the impedance between drain and source of the FET Q3), while when the R DS3 is large to be several tens kΩ, the change in the current is quite small to be negligible. However, when a usual GaAs MESFET is employed, the impedance thereof is relatively small to be several hundreds to 1kΩ, and the change in the current cannot be ignored. In the graph of figure 14, the drain terminal voltage of the FET Q3 changes by ΔV D3 = about 0.4V, and since the impedance R DS3 of the FET Q3 is about 2kΩ, the current changes by about ΔI3 ≒ 0.2 mA. By this influence, a difference of I1 - 12 = (1.77mA p- p ) - (1.64mA p-p ) = 0.13mA between the drain current amplitudes of the FET Q1 and the FET Q2 is generated, and a difference is generated in the output voltage amplitude at the both output terminals OUT and /OUT by the product of the current amplitude difference and the load resistance R1 or R2, i.e., (I1 - I2) R1 = 0.13mA x 600Ω = 78mV (this approximately coincides with the difference of 80mV between V OUT = 1.05 V p-p and V /OUT = 0.97 V p-p .) In the prior art differential amplifier circuit constructed as described above, the operation mechanism of turning on or off the FET Q1 and the FET Q2 is different between when the input signal voltage is high and when the input signal voltage is low. In the FET Q3 having a small drain-source impedance, there is generated a difference between the signal currents I1 and I2 flowing therethrough, thereby producing a difference in the output voltage amplitude. EP-A-0 354 557 discloses a differential amplifier having a single phase input and two phase output and comprises switching field effect transistors provided at the input side and at the reference side, respectively, forming a differential pair, and a field effect transistor serving as a constant current source for flowing a current of a sum of the currents flowing through said respective switching field effect transistors. It is an object of the present invention to improve a differential amplifier according to document EP-A-0 354 557 in such a way that output signals of equal amplitude are produced. This object, according to the present invention, is solved by the advantageous measures indicated in the characterizing part of claim 1. According to a first aspect of the present invention, a differential amplifier having a single phase input and two phase output includes, switching field effect transistors provided at the input side and at the reference side, respectively, forming a differential pair, a field effect transistor serving as a constant current source for flowing a current of a sum of the currents flowing through the respective switching field effect transistors, the values of the respective load impedances of the switching field effect transistors at the input side and at the reference side is set so that the relative value is smaller at the input side than at the reference side and the products of the value of the load and the value of the current flowing through the respective load are equal to each other at the both transistors. Therefore, the output voltages can be made equal to each other. According to a second aspect of the present invention, in differential amplifier, the loads of the switching field effect transistors at the input side and at the reference side comprise resistors having different resistance values at the input side and at the reference side respectively. Therefore, the output voltages can be made equal to each other. According to a third aspect of the present invention, in the differential amplifier, the load of the switching field effect transistor at the input side comprises a parallel circuit comprising a resistor having said predetermined resistance value and a serial circuit for adjusting the impedance of the load which comprises a capacitance and a resistance, and the load of the reference side switching field effect transistor comprises a resistor having a predetermined resistance value. Therefore, the respective loads of the respective switching FETs can be made equal with respect to DC and can be changed for signals, whereby the output voltages can be made equal to each other. According to a forth aspect of the present invention, in the differential amplifier, the load of the switching field effect transistor at the input side comprises a resistor having a predetermined resistance value, and the load of the switching field effect transistor at the reference side comprises a serial circuit comprising a resistor having the predetermined resistance value and an inductor for adjusting the impedance of the load. Therefore, the respective loads of the respective switching FETs can be made equal for DC, and can be changed for signals, whereby the output voltages can be made equal to each other. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a circuit diagram illustrating a differential amplifier having a single phase input and two phase output according to a first embodiment of the present invention. Figure 2 is a diagram illustrating a voltage waveform and a current waveform at the respective terminals in a differential amplifier having a single phase input and two phase output according to the first embodiment shown in figure 1. Figure 2 is a circuit diagram illustrating a differential amplifier having a single phase input and two phase output according to a second embodiment of the present invention. Figure 3 is a diagram illustrating a voltage waveform and a current waveform at respective terminals in a differential amplifier having a single phase input and two phase output according to the second embodiment shown in figure 2. Figure 5 is a circuit diagram illustrating a differential amplifier having a single phase input and two phase output according to a third embodiment of the present invention. Figure 6 is a diagram illustrating a voltage waveform and a current waveform at respective terminals in a differential amplifier having a single phase input and two phase output according to the third embodiment shown in figure 5. Figure 7 is a circuit diagram illustrating the prior art differential amplifier having a single phase input and two phase output. Figure 8 is a diagram illustrating a prior art voltage waveform and a current waveform at the respective terminals in a differential amplifier having a single phase input and two phase output shown in figure 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1. Figure 1 is a circuit diagram showing a first embodiment of the present invention. In the figure, the same reference numerals as those in figure 7 designate the same or corresponding elements. Reference numeral 10 designates the load resistance R3 which one end is connected to the cathode terminal of the diode D1, and which other end is connected to the drain terminal of the input side switching FET Q1. Next, a description is given of the operation. In the differential amplifier in this embodiment 1, in response to the difference of the current amplitude I1 and 12, the respective resistance values of the load of the input side switching FET Q1 and those of the reference side switching FET Q2 are changed which is caused by the fluctuation of the current ΔI3 at the constant current source, thereby the two phase signal output having equal amplitudes can be obtained. In figure 1, the value of the resistance R3 is given by I1 · R3 = I2 · R2. Figures 2(b)-2(d) show the timing waveforms of the voltages at the respective terminals and the currents flowing through the respective FETs in the differential amplifier shown in figure 1 which is also illustrated in figure 2(a) with parameters of respective elements. Reference characters and various lines in the respective graphs, the values of the input voltage and the respective voltage sources, and the input signal period are the same as those in figure 8. In the figure, when the value of the load R2 is 600 Ω, the current amplitudes of the current I1 and I2 flowing through the switching FETs Q1 and Q2 are 1.77 mA p-p and 1.64 mA p-p respectively, whereby R3 = I2 p-p · R2 /I1 p-p ≒ 550 Ω. As a result, the both phase output voltage amplitudes are approximately equal to each other, i.e., V OUT = 0.97 V p-p , V /OUT = 0.98 V p-p . As described above, in this embodiment 1, an equal output voltage amplitude can be obtained with changing the resistance values of the loads R1 and R2 for the switching FETs Q1 and Q2. Embodiment 2. Figure 3 is a circuit diagram showing a second embodiment of the present invention. In the figure, the same reference numerals as those in figure 7 designate the same or corresponding elements. Reference numeral 11 designates a capacitance C1 which one end is connected to the cathode terminal of the diode D1, reference numeral 12 designates the resistance R4 which one end is connected to the other end of the capacitance C1 and which other end is connected to the drain terminal of the FET Q1. Next, a description is given of the operation. In the fourth embodiment, since the values of the load resistance R2 and R3 are different, the difference arises between the drain bias voltages of the switching FETs Q1 and Q2. In this embodiment 5, however, since the resistance R4 is not visible with respect to DC bias, the load resistances of the FETs become R1 = R2, and the difference in the drain bias voltages does not arise. However, since at above a certain frequency the impedance Z C of the capacitance C1 become (2πfC) -1 ≒ 0 and the load of the FET Q1 is represented by the parallel circuit of the resistances R1 and R4, that is, R P = (R1 -1 + R4 -1 ) -1 , when the value of the resistance R4 is given such that I1 · R P = I2 · R2 for the current amplitudes I1 and I2, an output signal having an equal amplitude of the two phase can be obtained. Figures 4(b)-4(d) show the timing waveforms for the voltages at respective terminals and the currents flowing through the respective FETs in the differential amplifier shown in figure 3 which is also illustrated in figure 4(a) with parameters of respective elements. Reference characters and various lines in the respective graphs, the values of the input voltage and the respective voltage sources, and the input signal period are the same as those in figure 8. In the figure, capacitance C1 is 10 pF, and the resistance R4 is 6600 Ω. The output amplitudes become equal, i.e., V OUT = 1.00 V p-p and V /OUT = 0.99 V p-p , since the parallel circuit resistance R P = (600 -1 + 6610 -1 ) -1 ≒ 550 Ω. As described above, in this embodiment, the load resistance of the input side switching FET Q1 comprises a parallel circuit comprising a resistor having a predetermined resistance value and a serial circuit for adjusting the load resistance comprising a capacitance and a resistance, and the load of the reference side switching FET Q2 comprises a resistor having a predetermined resistance value, whereby the stable and equal output voltage amplitudes which are equal for DC and have different values of the load for signals can be obtained. Embodiment 3. Figure 5 is a circuit diagram showing a third embodiment of the present invention. In the figure, the same reference numerals as those in figure 7 designate the same or corresponding elements. Reference numeral 13 designates a load resistance R5 which one end is connected to the cathode terminal of the diode D1, and which other end is connected to the drain terminal of the FET Q1. Reference numeral 14 designates a load resistance R6 connected between the cathode terminal of the diode D1 and the other end of the inductor L4 which one end is connected to the drain terminal of the reference side FET Q2. Reference numeral 15 designates an inductor L4 connected between the load resistance R6 and the drain terminal of the FET Q2. Next, description is given of the operation. In the embodiment 3, the impedance Z L of the inductor L4 is 0 for DC bias and is ignored, and R5 = R6, whereby a difference does not arise in the drain bias voltage. At above a certain frequency, however, the load resistance of the FET Q2 is represented by (R6 + Z L ), the value of the inductor L4 is given such that I1 · R5 = I2 · (R6 + Z L ) for the current amplitudes I1 and I2, whereby the output signals having equal amplitudes for the two phases can be obtained. Figures 6(b)-6(d) show the timing waveforms of the voltages at the respective terminals and the currents flowing through the respective FETs in the differential amplifier shown in figure 5 which is also illustrated in figure 6(a) with parameters of respective elements. In the respective graphs, the reference characters and various lines, the input voltage and the values of the respective current voltage sources, and the input signal period designate the same as those in figure 8. In the figure, the resistance R5, R6 are 500 Ω, and the inductor L4 is 10 nH. The resintance of the RL serial circuit, RS = (500 + 125) = 625 Ω, and the two phase output amplitudes which are approximately equal, i.e., V OUT = 1.00 V p-p , and V /OUT = 0.98 V p-p are obtained. As described above, in this embodiment, the load of the input side switching FET Q1 comprises a resistor having a predetermined resistance value and the load of the reference side switching FET Q2 comprises an inductor for adjusting the load resistance connected in series to the resistor having a predetermined resistance value, whereby stable and equal output voltage amplitudes which are equal for DC and having different values of load for signals can be obtained.

Claims: A differential amplifier having a single phase input and two phase output comprising: switching field effect transistors (1, 2) provided at the input side and at the reference side, respectively, forming a differential pair; a field effect transistor serving as a constant current source (6) for flowing a current of a sum of the currents flowing through said respective switching field effect transistors (1, 2); characterized in that the values of the respective load impedances of said switching field effect transistors (1, 2) at the input side and at the reference side are set so that the respective value is smaller at the input side than at the reference side and the products of the value of the load impedance and the value of the current flowing through the respective load are equal to each other at both transistors (1, 2). The differential amplifier of claim 1, characterized in that the loads of said switching field effect transistors (1, 2) at the input side and at the reference side comprise resistors having different resistance values at the input side (10) and at the reference side (4), respectively. The differential amplifier of claim 1, characterized in that the load of said switching field effect transistor at the input side (1) comprises a parallel circuit comprising a resistor having a predetermined resistance value (3) and a serial circuit for adjusting the impedance of the load which comprises a capacitance (11) and a resistance (12), and the load of said reference side switching field effect transistor (2) comprises a resistor having said predetermined resistance value (3, 4). The differential amplifier of claim 1, characterized in that the load of said switching field effect transistor at the input side (1) comprises a resistor having a predetermined resistance value (13), and the load of said switching field effect transistor at the reference side (2) comprises a serial circuit comprising a resistor having said predetermined resistance value (14) and an inductor for adjusting the impedance of the load (15).

Inventors: MIYASHITA MIYO, MIYASHITA, MIYO, MIYASHITA, MIYO, C/O MITSUBISHI DENKI K. KAISHA
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: H03F3
IPC codes: H03F3, H03K5