Wide-band amplifier

Abstract: A differential amplifier is connected to a push-pull-type source follower circuit that receives biphase signals. The source follower circuit has a first set of FETs including a first source follower FET and a first current source FET and a second set of FETs including a second source follower FET and a second current source FET. A first coupling capacitance element connects the first current source FET and the second source follower FET, and a second coupling capacitance element connects the second current source FET and the first source follower FET. A diode or a resistor connects the source terminals of the first and second current source FETs to a voltage source.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide-band amplifier that assures satisfactory output characteristics over a wide operation frequency range of DC to several tens of gigahertz. 2. Description of the Related Art Fig. 4 is a circuit diagram showing a conventional wide-band amplifier capable of receiving biphase signals. In Fig. 4, reference numeral 1 denotes a differential amplifier. A level-shift first source follower circuit 2 shifts an output voltage of the differential amplifier 1 to a predetermined level. An impedance conversion second source follower circuit 3 is connected to the first source follower circuit 2. Further, symbols V SS is a voltage source; IN1, an input terminal; IN2, an inverted input terminal; OUT1, an output terminal; and OUT2, an inverted output terminal. In the differential amplifier 1, Tr1, Tr2 and Tr3 are field-effect transistors (hereinafter abbreviated as FETs), R1 and R2 are load resistors, and D1 is a level-shift diode. In the first source follower circuit 2, Tr4 and Tr5 are source follower FETs, Tr6 and Tr7 are current source FETs, and D2 and D3 are level-shift diodes. The stage of the source fol'lower FET Tr5 processes a signal that is inverted from a signal that is processed by the stage of the source follower FET Tr4. In the second source follower circuit 3, D4 and D5 are level-shift diodes, Tr8 and Tr9 are source follower FETs, and Tr10 and Tr11 are current source FETs. The stage of the source follower FET Tr9 processes a signal that is inverted from a signal that is processed by the stage of the source follower FET Tr8. The operation of the wide-band amplifier having the above configuration will be described below with assumptions that a current flowing through the FET Tr3 is denoted by I C and the resistors R1 and R2 have a resistance R L . When a high-level and low-level signals are respectively applied to the input terminal IN1 and the inverted input terminal IN2, the FET Tr1 is switched on and the FET Tr2 is switched off, so that the current I c flows through the FET Tr1. On the other hand, when low-level and high-level signals are respectively applied to the input terminal IN1 and the inverted input terminal IN2, the FET Tr1 is switched off and the FET Tr2 is switched on,-so that the current I C flows through the FET Tr2. A signal that is output from the drain terminal of the FET Tr1 or TR2 and supplied to the first source follower circuit 2 has an amplitude ΔV = R L x I C . The source follower circuit 2, actually the level-shift diodes D2 and D3, performs DC level shift on a signal that has been level-shifted by the differential amplifier 1, to obtain an output level that is required for the wide-band amplifier. Fig. 5A shows the first source follower circuit 2 being in a state that a gate voltage of the source follower FET Tr4 has just become a high level. At an instant that a high-level output signal is supplied from the differential amplifier 1 to the source follower FET Tr4, a gate-source voltage of the source follower FET Tr4 increases (by ΔV H ) and a drain current increases (by ΔI D1 ). If the internal resistance (source-drain resistance R DS ) of the current source FET Tr6 is sufficiently large, the drain current ΔI D1 flows into the gate terminal of the source follower FET Tr8 of the second source follower circuit 3, to charge up an input capacitance C t (gate-source capacitance C GS plus gate-drain capacitance C GD ) of the source follower FET Tr8. In general, a voltage V across a capacitance C is a time-integration of a current flowing therethrough, i.e., changes according to V = (1/C)∫idt. Therefore, a gate voltage of the source follower FET Tr8, i.e., a source voltage of the source follower FET Tr4 increases by ΔV H as time elapses, so that the gate-source voltage of the source follower FET Tr4 decreases by ΔV H . Thus, the injection current I D1 decreases to zero. In this case, to quickly switch the gate voltage of the source follower FET Tr8 from a low level to a high level (i.e., to reduce a rise time tr), the current ΔI D1 that is drawn from the source follower FET Tr4 needs to be increased. This can be attained by increasing the size of the FET Tr4. However, if the FET Tr4 were made too large, the load (i.e., input capacitance C t of the source follower FET Tr4) with respect to an output of the prior-stage differential amplifier 1 would become unduly large, deteriorating the switching operation of the differential amplifier 1. In the wide-band amplifier of Fig. 4, the load with respect to an output of the differential amplifier 1 is reduced by using the two-stage source follower circuits 2 and 3 in which the size of the source follower FETs of the second source follower circuit 3 is larger than that of the first source follower circuit 2. Source follower circuits of more than two stages may be employed to implement a very-high-speed amplifier. Fig. 5B shows the first source follower circuit 2 being in a state that the gate voltage of the source follower FET Tr4 has just become a low level. At an instant that a low-level output signal is supplied from the differential amplifier 1 to the source follower FET Tr4, the gate-source voltage of the source follower FET Tr4 decreases (by ΔV L ), so that the drain current decreases (by ΔI D2 ) and the internal resistance increases. Since a constant current is flowing through the current source FET Tr6, to cause the source voltage of the source follower FET Tr4 by ΔV L , the charge that has been stored in the input capacitance C t of the source follower FET Tr8 is released and a discharge current flows through the current source FET Tr6 as its drain current. To quickly switch the gate voltage of the source follower FET Tr8 from a high level to a low level (i.e., to reduce a fall time tf), the drain current of the current source FET Tr6 needs to be increased, resulting in increased power consumption of the entire circuit. The principle of operation of the second source follower circuit 3 is similar to that of the first source follower circuit 2. In the wide-band amplifier having the source follower circuits as described above, the increase of the number of stages of source follower circuits improves the high-speed switching characteristic, but deteriorates the bandwidth characteristic. To solve this problem, an amplifier has been proposed which employs a push-pull-type source follower circuit. Fig. 6 is a circuit diagram showing a conventional amplifier of such type as disclosed in Toshiki Seshita et al.: "20 GHz 8b Multiplexer Implemented with 0.5µm WN x /W-Gate GaAs MESFETs," IEEE ISSCC Digest of Technical Papers, pp. 172-173, Feb. 1994. The components given the same reference symbols in Figs. 4 and 6 are the same or corresponding ones. In Fig. 6, a differential amplifier 1 employs a level-shift resistor R4 in place of the level-shift diode D1 of Fig. 4. In a source follower circuit 4, current source FETs Tr6 and Tr7, source follower FETs Tr4 and Tr5, and diodes D2 and D3 are the same as those in the first source follower circuit 2 of Fig. 12. Gate voltages of the current source FETs Tr6 and Tr7 are given by a bias circuit including series resistors R5 and R6 and series resistors R7 and R8 that are provided between the ground and a voltage source V SS . Source voltages of the current source FETs Tr6 and Tr7 are given via diodes D4 and D5, respectively. A node ND1 at the source terminal of the source follower FET Tr4 is connected, via a capacitor C2, to a node ND4 at the gate terminal of the current source FET Tr7 of an inverted signal processing stage. A node ND2 at the source terminal of the source follower FET Tr5 of the inverted signal processing stage is connected, via a capacitor C1, to a node ND3 at the gate terminal of the current source FET Tr6. With the above push-pull-type configuration, when a low-level signal is applied to the source follower circuit 4, the current source FET Tr6 receives an inverted signal and therefore turns on, to increase a current flowing therethrough. Thus, the current source FET Tr6 more easily draws charge stored in an input capacitance of a next-stage circuit, resulting in a shorter signal fall time tf. Conversely, when a high-level signal is applied to the source follower circuit 4, the current flowing into the current source FET Tr6 is reduced. Since the current that is allowed to flow into the next-stage input capacitance is increased as much, a signal rise time tr can be shortened. Further, since the drive currents can be controlled by switching of the current source FETs Tr6 and Tr7, the high-speed switching characteristic can be improved with almost no increase of the power consumption of the bias circuit. Still further, because the bandwidth of the source follower circuit 4 is increased by virtue of the frequency characteristics of the coupling capacitors C1 and C2, the bandwidth of the entire wide-band amplifier can be increased. However, in the conventional wide-band amplifier using the push-pull-type source follower circuit 4, to make the gate voltages of the current source FETs Tr6 and Tr7 lower than their source voltages when they are turned off, the diodes D4 and D5 are connected to the source terminals of the respective FETs, negative feedback is effected by the internal resistances of the diodes D4 and D5. As a result, the switching characteristics of the current source FETs Tr6 and Tr7 are deteriorated, which in turn deteriorates the high-speed switching characteristic of the source follower circuit 4. SUMMARY OF THE INVENTION An object of the present invention is to provide a wide-band amplifier having a sufficiently wide bandwidth and sufficiently high switching speed as well as low power consumption which amplifier does not have a push-pull-type source follower circuit. This object is solved by the subject matter of claim 1. A further advantageous modification of the present invention is subject matter of claim 2. In the wide-band amplifier of claim 1, by providing the peaking circuit in series between the output terminals of an upstream source follower circuit and the input terminals of a downstream source follower circuit, the peaking circuit is caused to series-resonate with the input capacitance of the downstream source follower circuit, to increase only a high-frequency current, i.e., a current flowing into the input of a next-stage circuit such as a wide-band amplifier. No bias circuit for supplying gate voltages of the current source FETs is needed unlike the case of the push-pull-type source follower circuit. Therefore, the driving ability is improved without increase of the power consumption. The resonance frequency can be varied by changing the inductances of the peaking circuit. In the wide-band amplifier of claim 2 by making the resonance frequency of one peaking circuit different from that of at least one of the other peaking circuits, the frequency characteristic becomes flat between the two resonance frequencies. A flat voltage gain characteristic can be obtained in a desired frequency range by deviating the resonance frequencies of the respective wide-band amplifiers from each other. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing a wide-band amplifier according to a first embodiment of the invention; Fig. 2 is a circuit diagram showing a wide-band amplifier according to a second embodiment of the invention; and Fig. 3 is a graph showing frequency characteristics of the wide-band amplifier of Fig. 2; Fig. 4 is a circuit diagram showing a conventional wide-band amplifier; Figs. 5A and 5B illustrate charging/discharging in source following circuits of the wide-band amplifier of Fig. 4; and Fig. 6 is a circuit diagram showing a conventional wide-band amplifier having a push-pull-type source follower circuit. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Fig. 1 is a circuit diagram showing a first embodiment of the invention. In Fig. 1, reference numeral 1 denotes a differential amplifier. A level-shift first source follower circuit 2 shifts an output voltage of the differential amplifier 1 to a predetermined level. An impedance conversion second source follower circuit 3 is connected to the first source follower circuit 2. Reference numeral 6 denotes a peaking circuit. The differential amplifier 1, the first source follower circuit 2, and the second source follower circuit 3 are the same as the conventional ones shown in Fig. 4. This embodiment is characterized by the peaking circuit 6 that is provided between the output terminals of the first source follower circuit 2 and the input terminals of the second source follower circuit 3. In the differential amplifier 1, Tr1, Tr2 and Tr3 are FETs, R1 and R2 are load resistors, D1 is a level-shift diode, and R3 is a resistor. In the first source follower circuit 2, Tr4 and Tr5 are source follower FETs, Tr6 and Tr7 are current source FETs, and D2 and D3 are level-shift diodes. The stage of the source follower FET Tr5 processes a signal that is inverted from a signal that is processed by the stage of the source follower FET Tr4. In the second source follower circuit 3, D4 and D5 are level-shift diodes, Tr8 and Tr9 are source follower FETs, and Tr10 and Tr11 are current source FETs. The stage of the source follower FET Tr9 processes a signal that is inverted from a signal that is processed by the stage of the source follower FET Tr8. In the peaking circuit 6, an inductor L1 is provided between the inverted-signal output terminal (i.e., cathode electrode of the level-shift diode D2) of the stage of the source follower FET Tr4 and the inverted-signal input terminal (i.e., gate terminal of the source follower FET Tr8) of the second source follower circuit 3. An inductor L2 is provided between the non-inverted-signal output terminal (i.e., cathode electrode of the level-shift diode D3) of the stage of the source follower FET Tr5 and the non-inverted-signal input terminal (i.e., gate terminal of the source follower FET Tr9) of the second source follower circuit 3. A series resonance circuit is formed by the inductor L1 and the input capacitance of the source follower FET Tr8, so that a high-frequency current flowing through the source follower FET Tr8, i.e., a current flowing into the input of a next-stage circuit such as a wide-band amplifier is increased. Similarly, the inductor L2 increases a high-frequency current flowing through the source follower FET Tr9. By virtue of the use of the peaking circuit 6, which increases only a high-frequency current, no bias circuit for supplying gate voltages of the current source FETs is needed unlike the case of a push-pull-type source follower circuit. Therefore, the driving ability is improved without increase of the power consumption. The series resonance effect of the peaking circuit 6 is weaker if the output impedances of the portions of the prior-stage circuit to which the inductors L1 and L2 are connected are larger. Therefore, the circuit configuration of this embodiment, in which the peaking circuit 6 is connected to the output terminals of the impedance-reduced first source follower circuit 2 rather than provided between the output terminals of the differential amplifier 1 and the input terminals of the first source follower circuit 2, maximizes the series resonance effect. The resonance frequency of the peaking circuit 6 can be varied by changing the inductance of the inductors L1 and L2. Further, the bandwidth of the wide-band amplifier can be increased by setting the resonance frequency of the peaking circuit 6 close to the high-frequency cut-off frequency of the wide-band amplifier. Embodiment 2 Fig. 2 is a circuit diagram showing a second embodiment of the invention. In this embodiment, as shown in Fig. 2, a first wide-band amplifier 7 having a peaking circuit 6 that consists of inductors L1 and L2 and a second wide-band amplifier 12 having a peaking circuit 11 that consists of inductors L3 and L4 are connected to each other in series. The first wide-band amplifier 7 includes a differential amplifier 1, a level-shift first source follower circuit 2, an impedance conversion second source follower circuit 3, and the peaking circuit 6 provided between the output terminals of the first source follower circuit 2 and the input terminals of the second source follower circuit 3. The second wide-band amplifier 12 includes a differential amplifier 8, a level-shift first source follower circuit 9, an impedance conversion second source follower circuit 10, and the peaking circuit 11 provided between the output terminals of the first source follower circuit 9 and the input terminals of the second source follower circuit 10. An inductance L f of the inductors L1 and L2 of the resonance circuit 6 and its resonance frequency f f are made different from an inductance L s of the inductors L3 and L4 of the peaking circuit 11 and its resonance frequency f s ; that is, a relationship f f ≠ f s is established. Fig. 3 shows bandwidth characteristics of the first and second wide-band amplifiers 7 and 12. More specifically, a solid line A represents a frequency characteristic the voltage gain of the entirety of the first and second wide-band amplifiers 7 and 12, a chain line C represents a frequency characteristic of the voltage gain of only the first wide-band amplifier 7, and a dashed line B represents a frequency characteristic of the voltage gain of only the second wide-band amplifier 12. As seen from Fig. 3, the frequency characteristic (solid line A) is flat between a resonance frequency f f (≒ 4 GHz) of the first wide-band amplifier 7 (chain line C) and a resonance frequency f s (≒ 5.5 GHz) of the second wide-band amplifier 12 (dashed line B). In this manner, a flat voltage gain characteristic can be obtained in a desired frequency range by deviating the resonance frequencies of the respective wide-band amplifiers from each other.

Claims: A wide-band amplifier, comprising: a plurality of source follower circuits receiving biphase signals and connected in series, each of the source follower circuits comprising a first set of FETs including a first source follower FET and a first current source FET and a second set of FETs including a second source follower FET and a second current source FET; and a peaking circuit provided in series between output terminals of an upstream source follower circuit and input terminals of a downstream source follower circuit, the peaking circuit comprising inductors that series-resonate with the downstream source follower circuit. The wide-band amplifier according to claim 1, wherein the wide-band amplifier comprises at least two peaking circuits, and one of the peaking circuits has a different resonance frequency than at least one of the other peaking circuits.

Inventors: MIYASHITA MIYO, SHIMADA MASAAKI, YAMAMOTO KAZUYA, MIYASHITA, MIYO, SHIMADA, MASAAKI, YAMAMOTO, KAZUYA, Miyashita, Miyo, Hikari Micro-ha Device, Shimada, Masaaki, Hikari Micro-ha Device, Yamamoto, Kazuya, Hikari Micro-ha Device
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1996
Main classification: H03F3
IPC codes: H03K19, H03F1, H03F3