Single Stage Balanced Voltage Amplifier

Abstract

A balanced voltage amplifier is disclosed as comprising a single stage containing three pairs of vacuum tube triodes configured to amplify two input signals (+INPUT, −INPUT) and to generate two output signals (+OUTPUT, −OUTPUT). The balanced voltage amplifier offers high voltage gain, wide bandwidth and low output impedance. Local feedback may be applied between the outputs and the second pair of triodes. Overall feedback may be applied between the outputs and the first pair of triodes. If local or overall feedback is used, it will further broaden the bandwidth, lower the output impedance and improve the overall balancing.

Description

Embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:

FIG. 1 shows a circuit diagram of a single stage balanced voltage amplifier according to a first embodiment of the present invention.

FIG. 2 shows a circuit diagram of a single stage balanced voltage amplifier according to a second embodiment of the present invention, being a modified arrangement of the arrangement shown in FIG. 1, in which local feedback is applied between the output and the second pair of vacuum tubes.

FIG. 3 shows a circuit diagram of a single stage balanced voltage amplifier according to a third embodiment of the present invention, being a further modified arrangement of the arrangement shown in FIG. 1, in which overall feedback is applied between the output and the first pair of vacuum tubes.

FIG. 4 shows a circuit diagram of a conventional balanced amplifier, namely a differential amplifier.

FIG. 5 shows a circuit diagram of an improved conventional balanced voltage amplifier, namely a cascode differential amplifier.

As shown in FIG. 1, a first embodiment of a balanced voltage amplifier according to the present invention is realized in the form of a single-stage amplifier containing three pairs of vacuum tubes V101a, V101b; V102a, V102b; V103a, V103b. The vacuum tubes V101a, V101b; V102a, V102b; V103a, V103b can be any of the commonly used small signal vacuum tube triodes, such as 12AT7, 12AU7, 12AX7, 6DJ8, 6SN7, 6SL7, 6H30P, and the like. However, if low output impedance is a key wanted feature, vacuum tubes with low plate impedance, such as 6DJ8 and 6H30P, should be used.

The input signals (+INPUT, −INPUT) are fed to the first pair of vacuum tube triodes V101a, V101b. The outputs of these vacuum tube triodes V101a, V101b are then fed to the cathodes of the second pair of triodes V102a, V102b. The resistor networks formed by resistors R103, R104, R105, R106 set up a correct DC biasing voltage at the grid of the second pair of triodes V102a, V102b. The output of the second pair of triodes V102a, V102b are taken from the plates and fed to the cathodes of the third pair of triodes V103a, V103b via resistors R107, R108, R109, R110. The purpose of resistors R109, R110 is to improve the DC biasing stability of the whole balanced voltage amplifier.

It should be noted that bypass capacitors may be connected from the grid of triodes V102a, V102b to ground to filter out noises generated from the power supplies that might be amplified by the triodes. However, as long as the voltage supplies +VS101 and −VS102 are properly regulated and filtered, bypass capacitors may be omitted.

The third pair of triodes V103a, V103b are self-biased by the voltage potential, which is created by cathode resistors R107, R108, and is fed to the grid of the triodes V103a, V103b. The outputs are taken from the cathodes of the triodes V103a, V103b via coupling capacitors C101, C102 respectively. As the output impedance at the cathode of a triode is low, the output impedance of the overall balanced voltage amplifier is kept low. The resistors R111, R112 set the overall signal gain. A small value variable resistor VR1 is optionally used for fine adjusting the overall balancing, although it should be understood that such a variable resistor is not an essential feature.

The constant current source CS1 may be easily implemented by the use of solid-state semiconductor devices such as junction gate field-effect transistor (JFET), bipolar junction transistor (BJT) or metal-oxide-semiconductor field-effect transistor (MOSFET) with complementary components zener diode and resistors. In some practical situations, a simple resistor can replace the current source. However, a constant current source will generally improve the common-mode-rejection-ratio (CMRR) of the overall balanced voltage amplifier.

If some local feedback is used, the bandwidth will be broadened and the output impedance may be further lowered. FIG. 2 reveals such an arrangement, according to a second embodiment of the present invention. It can be seen that this circuit arrangement also includes three pairs of vacuum tube triodes V201a, V201b; V202a, V202b; V203a, V203b. The outputs of the third pair of triodes V203a, V203b are fed to the gates of the second pair of triodes V202a, V202b via RC networks C203, R213; C204, R214.

FIG. 3 shows a further circuit arrangement of a balanced voltage amplifier according to a third embodiment of the present invention, in which overall feedback is applied to the amplifier. Again, this circuit arrangement also includes three pairs of vacuum tube triodes V301a, V301b; V302a, V302b; V303a, V303b. It can be seen that the outputs of the third pair of triodes V303a, V303b are fed to the crossed cathodes of the first pair of triodes V301a, V301b via RC networks C305, R315; C306, R316. With the use of overall feedback, it will not only broaden the overall bandwidth and lower the output impedance, but will also reduce the sensitivity of parameters change of vacuum tubes V301a, V301b; V302a, V302b; V303a, V303b due to aging. The overall balancing can be also improved.

With the present invention, as shown in FIGS. 1 to 3, new balanced voltage amplifiers with high signal gain and low output impedance can be formed by three pairs of vacuum tube triodes confined in a single stage.

It should be understood that the above only illustrates examples whereby the present invention may be carried out, and that various modifications and/or alterations may be made thereto without departing from the spirit of the invention.

It should also be understood that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any appropriate sub-combinations.

Claims

1. A balanced voltage amplifier including a single stage containing at least a first, a second and a third pair of vacuum tube triodes for amplifying two input signals and generating two output signals.

2. A balanced amplifier according to claim 1 wherein said input signals are fed to the grids of said first pair of vacuum tube triodes, and a grid resistor is connected to ground on each triode to maintain proper DC biasing.

3. A balanced amplifier according to claim 1 wherein outputs of said first pair of triodes taken from the plates are fed to the cathodes of said second pair of triodes, further including a plurality of resistors to set up a pre-determined DC biasing voltage at the grid of said second pair of triodes.

4. A balanced amplifier according to claim 3 wherein bypass capacitors are connected from the grid of said second pair of triodes to ground.

5. A balanced amplifier according to claim 2 wherein outputs of said second pair of triodes taken from the plates are fed to the cathodes of said third pair of triodes via two series resistors.

6. A balanced amplifier according to claim 5 wherein each of said third pair of triodes is self-biased by a voltage potential created by a first resistor in series from the cathode, and fed to the grid of each of said third pair of triodes.

7. A balanced amplifier according to claim 6 further including a second resistor in series with said first resistor.

8. A balanced amplifier according to claim 7 wherein outputs are taken from the cathodes of said third pair of triodes via a plurality of coupling capacitors.

9. A balanced amplifier according to claim 8 wherein two fixed resistors and a variable resistor are connected between the cathodes of said first pair of triodes with the variable resistor placed in the middle.

10. A balanced amplifier of claim 9 wherein said two fixed resistors are provided for determining the voltage gain.

11. A balanced amplifier according to claim 9 wherein said variable resistor is of a small value and is provided for fine adjusting the balancing of the amplifier.

12. A balanced amplifier according to claim 9 wherein a resistor or a constant current source is connected from the middle tap of said variable resistor to a negative power supply.

13. A balanced amplifier according to claim 12 wherein said constant current source comprises a JFET, MOSFET, BJT, a vacuum tube triode, a pentode with complementary zener diode and resistors, or a resistor.

14. A balanced amplifier according to claim 1 wherein local feedback is applied between the outputs of said third pair of triodes and the grids of said second pair of triodes.

15. A balanced amplifier according to claim 14 wherein said local feedback is provided by an RC network.

16. A balanced amplifier according to claim 1 wherein overall feedback is applied between the outputs of the third pair of triodes and the crossed cathodes of said first pair of triodes.

17. A balanced amplifier according to claim 16 wherein said overall feedback is provided by an RC network.

18. (canceled)

19. A balanced amplifier according to claim 2 wherein outputs of said first pair of triodes taken from the plates are fed to the cathodes of said second pair of triodes, further including a plurality of resistors to set up a predetermined DC biasing voltage at the grid of said second pair of triodes.