US3163827A - Cathode-follower and emitter-follower circuits - Google Patents

Cathode-follower and emitter-follower circuits Download PDF

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US3163827A
US3163827A US218653A US21865362A US3163827A US 3163827 A US3163827 A US 3163827A US 218653 A US218653 A US 218653A US 21865362 A US21865362 A US 21865362A US 3163827 A US3163827 A US 3163827A
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transistor
emitter
current
collector
base
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US218653A
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Kandiah Kathirkamathamby
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UK Atomic Energy Authority
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UK Atomic Energy Authority
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F5/00—Amplifiers with both discharge tubes and semiconductor devices as amplifying elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B11/00—Communication cables or conductors
    • H01B11/18—Coaxial cables; Analogous cables having more than one inner conductor within a common outer conductor
    • H01B11/1834—Construction of the insulation between the conductors
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/34—Negative-feedback-circuit arrangements with or without positive feedback
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/26—Push-pull amplifiers; Phase-splitters therefor
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/30—Single-ended push-pull [SEPP] amplifiers; Phase-splitters therefor
    • H03F3/3083—Single-ended push-pull [SEPP] amplifiers; Phase-splitters therefor the power transistors being of the same type

Definitions

  • the voltage gain of a simple cathode follower can be made to approach unity, which is desirable from the point of view of reducing the efiective input capacity, provided that the nett cathode load is of high impedance.
  • a low output impedance can be obtained by using a White cathode follower but, in its normal form the low output impedance of the white cathode follower does not extend down to zero frequency because of the AC. coupling.
  • Emitter followers are also used to match high imped ance signal sources to low impedance loads.
  • the output resistance is approximately s c E where r is the emitter resistance under common-base conditions, ,8 is the common emitter current gain and r is the resistance of the signal source.
  • the output resistance is lower when two or more emitter followers are used in cascade but it can never be less than the r of the last stage, which is usually in theregion of a few ohms, unless power transistors are used with a large emitter current.
  • the present invention seeks to achieve a gain closely approaching unity and a low output impedance from zero up to a very high frequency.
  • the transistors referred to as the first and second transistors are, of course, additional to the emitter follower transistor if one is used. They may be p-n-p transistors, and the collector load of the first trmsistor may be a resistor or a further transistor connected in a constantcurrent circuit.
  • the first and second transistors may be n-p-n transistors, and the cathode load maybe a further transistor connected in a constant-current circuit.
  • FIGS. 1, 2 and 3 are circuit diagrams of cathode-follower circuitsembodylarger than R1 so that 13R RL ing the invention and FIGS. 4 to 8 are circuit diagrams of emitter-follower circuits embodying the invention.
  • the cathode current of a valve V1 flows to the emitter of a first p-n-p transistor 32 and thence through the collector load R1. Any change in the collector current of 12 is fed to the base of a second p-n-p transistor J3 and fed back from J3 emitter to 12 base in a sense to oppose the change. The effect is to maintain the collector current of J2, and hence the cathode current of V1, substantially constant. Hence the emitterbase voltageof J2 and the grid-cathode voltage of V1 remain constant and the output from J3 emitter closely follows the input to the grid of V1.
  • the transistor 13 drives the output load RL.
  • the function of R2 is to provide current for positive excursions of the output.
  • Equation 1 then reduces to:
  • a constant-current collector load for J2 is provided by the n-p-n transistor J4, whose emitter current is defined by the potentiometer R5, R6 and the emitter resistor R7.
  • the output voltage variations are fed to the anode of V1 through the gas discharge diode V2, thereby maintaining constant the anode-cathode voltage of V1 and hence the anode current. Zener diodes of the appropriate voltage can be used in place of V2 with advantage, because of their lower impedance.
  • Equation 1 simplifies to Where R is the value of the load resistance in parallel with the resistance R4- which is in series with the anode of V1.
  • the output'resistance R is given by R approx. R (8)
  • R is the value of the load resistance in parallel with the resistance R4- which is in series with the anode of V1.
  • the output'resistance R is given by R approx. R (8)
  • R is the value of the load resistance in parallel with the resistance R4- which is in series with the anode of V1.
  • the output'resistance R is given by R approx. R (8)
  • a ply voltage need only be slightly greater than the maximum negative excursion of the signal, because the collector of J4 need only be a few volts more positive than this supply.
  • the emitter current of J3 for positive output excursions is supplied via V2.: Typical component values in the circuit of FIG.
  • V1 is an RCA Nuvistor 6CW4 and V2 has a running voltage of about 100v.
  • FIG. 3 shows a modification of the circuit of FIG. 2 for use with large positive excursions of load current.
  • the p-n-p transistors J2, J3 are replaced by n-p-n transistors J2, J3.
  • V1 has a constant-current cathode load comprising an n-p-n transistor J5 and associated resistors R12, R13 and R14. Any change in the cathode current of V1 therefore appears as a change of equal magnitude in the opposite sense in the emitter current of J2, whose collector current is fed back to the base or J2 via J3 as before to oppose the change.
  • a constant-current load for the collector of I2 is unnecessary, since the positive supply voltage E1 is large (say 150 v.) and enables R8 to be large.v
  • V1 and V2 are as in FIG. 2.
  • E5 may be 15 volts positive and E4 15 volts negative.
  • the transistor J2 or J2 may require a larger collector voltage than that provided in the circuits shown. This can be achieved by inserting a Zener diode of suitable voltage in series with the emitter of the output transistor J3 or J3.
  • the circuits of FIGS. 2 and 3 can be particularly useful because of the extremely low input capacitance that can be achieved. Since the anode and cathode of V1 follow the input signal, the grid-anode and grid-cathode capacitances are effectively reduced by a'factor of about R/R The same reduction in the capacitance between the, input and various shields can be obtained by connecting these shields to the output. In practice it may be possible to reduce the input capacitance to a value considerably smaller than 1 p F. even when the load resistance at the output is as small as ohms.
  • FIG. 4 A basic emitter-follower circuit of the invention is shown in FIG. 4 in which an n-p-n transistor J1 connected as an emitter-follower takes the place of the valve V1.
  • the current through both the transistor J1 and the pn-p transistor J2 is approximately E2/R1 and that through the p-n-p transistor J3 is approximately E1/R2 (neglect ing the current through the load RL) under static conditions.
  • Most of the load current flows through J3.
  • r emitter output resistance .of J3 with common-base
  • the input resistance is plfigR shunted by r l where 13 is the common emitter current gain and r l is the collectorbase resistance of J 1.
  • the output resistance is where R is the output resistance of the signal source.
  • the load resistance is 100 ohms
  • the input resistance will be 250K ohms shunted by r l assuming [3 :5 :50.
  • the current through J1 and J2 is fixed by a constant-current load which can take any of the well known forms.
  • this load is provided by the n-p-n transistor J4 whose emitter current is defined by the potentiometer R5, R6 and the emitter resistor R7
  • the Zener diode D ensures that the voltage between base and emitter of J1 remains constant so that r l of the first transistor no longer becomes an appreciable component of the input resistance.
  • the current through R4 in excess of that through J1 flows through J3.
  • the integrating time-constant R3C1 ensures that the system remains stable, it being necessary also to use as J2 as a transistor with a cut-ofi frequency much higher than that of J1 and J3. With J1 and J3 having cut-otf frequencies of about 1 or 2 -mc./s.
  • FIG. 6 shows a further embodiment (analogous to the arrangement shown in FIG. 3) in which J2 is an n-p-n transistor, Constant-current loads 11 and 12 are preferably provided for J1 and J2 respectively, but resistors may be adequate in some applications.
  • This circuit has two advantageous characteristics. Firstly, when the input voltage at the base'of J1 is zero the output voltage is Zero since the emitter-base potentials of J1 and J2' are in opposite directions and so substantially cancel out; hence the load current is zero. Secondly, because these potentials are in opposite directions, variations therein due to temperature changes similarly cancel out, so reducing drift. By contrast the emitter-base potentials of J1 and J2 in FIGS.
  • FIG. 7 Another form of the circuit which is particularly useful for driving a low value of load resistance with peak voltages approaching the values of the power supply lines is shown in FIG. 7.
  • the collector of J1 is taken to the base of a p-n-p transistor J 4 whose emitter is connected to the positive supply line and whose collector is connected to the output.
  • FIGS. 4 and 5 the their current (gains.
  • FIG. 8 shows a circuit for use where supply lines of both polarities are not available.
  • One end of the load RL (which might be a loudspeaker) is connected to the output of a circuit as in FIG. 7 comprising J1, J2, J3, and J4.
  • the other end of RL is connected to the output of an identical referencecircuit comprising J11, J22, J33
  • the input voltage V is applied between the bases of J]. and Jill, Whose static potential is determined by a tapping on a'common potential divider R19, R11 connected to J11 base.
  • the main virtues of the circuits are high input impedance, low output impedance and a gain very close to unit. All the above characteristics are maintained from DC. up to a frequency whose limit is set purely by the cut-off frequencies of the transistors.
  • n-p-n transistor J1 can be replaced by a p-n-p type if corresponding modifications are made to the remainder of :the circuit.
  • J2 and J3 would then become n-p-n type transisters.
  • a circuit for matching a high impedance signal source to a low impedance load comprising a thermionic valve which has an anode, a cathode and a control grid and which is connected as a cathode-follower, an input connection to said control grid, first and second supply lines, a connection from said anode to the first supply line, a first junction transistor having its emitter connected to said cathode such that any change in the cathode current of said valve appears as a change of equal magnitude in the emitter current of the first transistor, a resistive collector load for the first transistor connected between the collector electrode of the first transistor and the second supply line, a second junction transistor, connections between the collector of the first transistor and the base of the second transistor and between the emitter of the second transistor and the base of the first transistor such that any change in the'collec-t-or current of the first transistor is fed back to oppose the change, a connection between the collector of the second transistor and the second supply line, a resistive connection between the base of the
  • a circuit for matching a high impedance signal source to a low impedance load comprising a thermionic valvewhich has an anode, a cathode and a control grid and which i connected as a cathode-follower, an input connection to said control grid, first and second supply lines, a resistive connection from said anode to the first supply line, a first junction transistor having its emitter connected to said cathode such that any change in the which is connected in series with resistance between said cathode and the, second supply line, and the base of which is connected to -a point on a potentiometer connected between the second supply line and earth, a constantvoltage device connected between said anode and the base of the first transistor such that the anode current of said valve is maintained substantially constant, and an output connection from the emitter of the second transistor.
  • a circuit for matching a high impedance signal source to a low impedance load comprising a first junction transistor which is connected as an emitter-follower, an input connection to the base of the first transistor, first and second supply lines, a connection from the collector of the first transistor to the first supply line, a second transistor having its emitter connected to the emitter of the first transistor such that any change in the emitter current of the first transistor appears as a change of equal magnitude in the emitter current of the second transistor, a re sistive collector load for the second transistor connected between the collector electrode of the second transistor and the second supply line, a third junction transistor, connections between the collector of the second transistor and the base of the third transistor and between the emitter of the third transistor and the base of the second transistor such that any change in the collector current of the second transistor is fed back to oppose the change, a connection between the collector of the third transistor and the second supply line, a resistive connection between the base of the second transistor and the first supply line, and
  • a circuit for matching a high impedance signal source to a low impedance load comprising a first junction transistor which is connected as an emitter-follower, an input connection to the base of the first transistor, first and second supply lines, a resistive connection from the collector of the first transistor to the first supply line, a second transistor having its emitter connected to the emitter of the first transistor such that'any change in the emit ter current of the first transistor appears as a change of equal magnitude in the emitter current of the second transistor, a third junction transistor, connections between the collector of the second transistor and the base of the third transistor and between the emitter of the third transistor and the base of the second transistor such that any change in the collector current of the second transistor is fed back to oppose the change, a constant-current network formed by a fourth junction transistor the emittercollector path of which is connected in series with resistance between the collector of the second transistor and the second supply line, and the base of which is connected to a point on a pot

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  • Power Engineering (AREA)
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US218653A 1961-08-22 1962-08-22 Cathode-follower and emitter-follower circuits Expired - Lifetime US3163827A (en)

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GB30279/61A GB973753A (en) 1961-08-22 1961-08-22 Improvements in or relating to cathode-follower and emitterfollower circuits

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US (1) US3163827A (enrdf_load_stackoverflow)
DE (1) DE1186512B (enrdf_load_stackoverflow)
GB (1) GB973753A (enrdf_load_stackoverflow)
NL (1) NL282327A (enrdf_load_stackoverflow)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3268828A (en) * 1962-11-02 1966-08-23 Burroughs Corp Amplifier with constant amplitude output
US3296540A (en) * 1963-05-07 1967-01-03 John C May Transistor back bias
US3421102A (en) * 1965-06-10 1969-01-07 Tektronix Inc Direct coupled temperature compensated amplifier
US3451001A (en) * 1966-08-15 1969-06-17 Bunker Ramo D.c. amplifier
US3543173A (en) * 1969-01-21 1970-11-24 Bendix Corp Class b power amplifier
US4021747A (en) * 1974-10-29 1977-05-03 Tokyo Shibaura Electric Co., Ltd. Signal amplifier circuit using a pair of complementary junction field effect transistors

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL7104636A (enrdf_load_stackoverflow) * 1971-04-07 1972-10-10
JPS5221751A (en) * 1975-08-12 1977-02-18 Toshiba Corp Voltage follower circuit

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3268828A (en) * 1962-11-02 1966-08-23 Burroughs Corp Amplifier with constant amplitude output
US3296540A (en) * 1963-05-07 1967-01-03 John C May Transistor back bias
US3421102A (en) * 1965-06-10 1969-01-07 Tektronix Inc Direct coupled temperature compensated amplifier
US3451001A (en) * 1966-08-15 1969-06-17 Bunker Ramo D.c. amplifier
US3543173A (en) * 1969-01-21 1970-11-24 Bendix Corp Class b power amplifier
US4021747A (en) * 1974-10-29 1977-05-03 Tokyo Shibaura Electric Co., Ltd. Signal amplifier circuit using a pair of complementary junction field effect transistors

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GB973753A (en) 1964-10-28
DE1186512B (de) 1965-02-04
NL282327A (enrdf_load_stackoverflow)

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