Abstract:
A single stage differential amplifier is disclosed as comprising a pair of vacuum tube triodes for amplifying two input signals and generating two output signals. The differential amplifier has DC self-biasing ability and grid-to-cathode over-voltage protection for directly coupling from the outputs of another differential amplifier. By possessing these unique features, this differential amplifier becomes an important building block in forming a balanced amplifier by cascading multi differential amplifiers in a directly coupled fashion.

Description:
BACKGROUND 
   As mentioned in U.S. Pat. No. 7,304,535, a vacuum tube balanced audio power amplifier normally contains three amplifying stages, namely an input stage, a second stage and an output stage. These are shown in FIGS. 3, 4, 5A, 5B, 6A, 6B, 9 and 10 of U.S. Pat. No. 7,304,535. Differential amplifiers of various kinds can be used to form the input and second stages. A commonly used vacuum tube differential amplifier is shown in  FIG. 7  hereof. This differential amplifier can be used to form the first two stages of a conventional vacuum tube balanced audio power amplifier, as depicted in  FIG. 8  hereof. 
   It is clear from  FIG. 8  hereof that the input stage is a differential amplifier constructed by two triodes in the exact same form as shown in  FIG. 7 . Resistors R 1  and R 2  connect the grid to ground so that correct DC biasing can be set up for the tubes T 1   a  and T 1   b . The outputs taken from the plates of vacuum tubes T 1   a , T 1   b  are directly coupled respectively to the grids of vacuum tubes T 5   a , T 5   b  of the second stage via series resistors R 36  and R 37 . The second stage is also a differential amplifier in the exact same form. The outputs taken from the plates of the vacuum tubes T 5   a  and T 5   b  of the second stage are R-C coupled to the output stage via capacitors C 10 , C 11  and resistors R 25  and R 26 . The output stage consists of a pair of beam power tubes T 3 , T 4  and output-matching transformer OPT. 
   Unlike small signal solid-state semiconductor transistors, in which the DC current gain (i.e., h FE , or sometimes referred to as DC amplifying factor) is very close for the same type of transistors, the DC amplifying factor of small signal vacuum tubes of the same type can differ immensely. It is because the advanced technologies of solid-state semiconductor fabrication allow the parameters of transistors to be tightly controlled. However, manufacturing of vacuum tube still relies on the skills of production workers in winding and aligning the wires and metal plates. 
   The differences in vacuum tube DC amplifying factor make it very difficult for vacuum tube amplifying stages to be directly coupled while maintaining the correct DC biasing for subsequent stages. It is obvious that in direct coupling of amplifying stages, the mismatched DC biasing voltages created from the first stage will be passed to the subsequent stages so that an even bigger mismatch of DC biasing voltages is created. As a result, the two vacuum tubes of a differential amplifier in a subsequent stage have to face two very different DC biasing voltages. Therefore, this leads to different biasing currents, signal voltage swings, output impedances and distortion levels at the two outputs of the differential amplifier in the subsequent stage. Hence the desired balancing properties of a vacuum tube balanced audio power amplifier can no longer be maintained. 
   In such a scenario, a common practice is to choose well-matched vacuum tubes for direct coupling applications. However, it is sometimes impractical and time consuming to screen the tubes. Even if well-matched vacuum tubes are used, as the tubes get aged after a period of operation, the mismatch will eventually resurface as some tubes deteriorate faster than others. Therefore, tube aging again creates mismatch of DC biasing. Another conventional method is to use R-C coupling rather than direct coupling between amplifying stages. This can totally eliminate the mismatched DC biasing problem passing from previous stages as DC voltages are completely blocked by the coupling capacitors. However, it is well known that an amplifier formed by R-C coupling of amplifying stages generally has poorer low frequency response than one formed by direct coupling. Poor low frequency response is not desirable in audio application. 
   The aim of this invention is to provide a new differential amplifier such that the use of well-matched vacuum tubes is no longer an absolute necessity for direct coupling applications. This invention will at least reduce the DC biasing mismatch to an acceptable level for most direct coupling applications. In addition, a grid-to-cathode over-voltage protection is included. 
   According to the present invention, there is provided a single stage differential amplifier including a pair of vacuum tube triodes for amplifying two input signals and generating two output signals, wherein said input signals are fed to the grids of said pair of vacuum tube triodes, and a pair of two series resistors on each grid is cross-connected to two separate junctions formed by a pair of two series resistors, respectively, such that the latter pair of series resistors are connected together with a constant current source connected to a negative power supply. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a detailed circuit arrangement of a differential amplifier with DC self-biasing according to a first preferred embodiment of the present invention; 
       FIG. 2  is a detailed circuit arrangement of a simplified differential amplifier with DC self-biasing but without degenerated resistors according to a second preferred embodiment of the present invention; 
       FIG. 3  is a detailed circuit arrangement of a simplified differential amplifier with DC self-biasing, with degenerated resistors but without bypassing capacitor, according to a third preferred embodiment of the present invention; 
       FIG. 4  is a detailed circuit arrangement of a vacuum tube balanced audio power amplifier employing the differential amplifier of  FIG. 1  in the second stage; 
       FIG. 5  is a detailed circuit arrangement of a vacuum tube balanced audio power amplifier based on the arrangement of  FIG. 4 , with the grid-to-cathode over-voltage protection circuit formed by diodes D 1 -D 2  and zener diodes ZD 1 -ZD 2 ; 
       FIG. 6  is a detailed circuit arrangement of the new differential amplifier with DC self-biasing and grid-to-cathode over-voltage protection; 
       FIG. 7  is a detailed circuit arrangement of a conventional differential amplifier; and 
       FIG. 8  is a detailed circuit arrangement of a conventional vacuum tube balanced audio power amplifier. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIG. 1 , a first preferred embodiment of a differential amplifier according to the present invention is realized in the form of a single-stage amplifier containing one pair of vacuum tube triodes T 2   a  and T 2   b . The vacuum tubes T 2   a , T 2   b  can be any of the commonly used small signal dual triodes, such as 12AT7, 12AU7, 12AX7, 6922, 6DJ8, 6SN7, 6SL7, 6H30P and the like. We assume that the inputs of this differential amplifier are directly coupled to the outputs of the previous amplifying stage, which is a conventional differential amplifier of  FIG. 7 . 
   It can be seen that the triodes T 2   a  and T 2   b  amplify two input signals (+Input, −Input) and generate two output signals (+Output, −Output). The output signals are taken from the plates of the vacuum tube triodes T 2   a , T 2   b . The input signals are fed to the grids of the triodes T 2   a , T 2   b , and a pair of two series resistors R 11 -R 12 , R 13 -R 14  are cross-connected to two separate junctions formed by a pair of two series resistors R 16 -R 17 , R 18 -R 19  respectively, such that the pairs of series resistors R 16 -R 17 , R 18 -R 19  are connected together with a constant current source CS 2  connected to a negative power supply −VS 5 . The constant current source CS 2  may be a junction gate field-effect transistor (JFET), a metal-oxide-semiconductor field-effect transistor (MOSFET), a bipolar junction transistor (BJT), a vacuum tube triode, a pentode with complementary diodes, zener diodes and resistors, or a resistor. 
   Capacitors C 1 , C 2  are connected to ground at the junctions between the two series grid resistors R 11 -R 12 , R 13 -R 14  on each grid. The pair of series resistors R 16 -R 17 , R 18 -R 19  are connected to the cathodes of the triodes T 2   a  and T 2   b  via a separate cathode series resistors, R 15 , R 20 . A capacitor C 3  is connected to the junctions formed between the cathode series resistors R 15 , R 20  and the pair of two series resistors R 16 -R 17 , R 18 -R 19 . This circuit arrangement also includes a pair of plate resistors R 21 , R 22 , connected to a positive power supply +VS 2 . 
   It is clear that the grids of the vacuum tube T 2   a  and T 2   b  carry both input signals and DC biasing voltages passed from the previous amplifying stage. Hence, we should examine the amplifier from two different aspects: (i) small signal point of view and, (ii) DC biasing point of view. 
   From the small signal point of view, the operation of the differential amplifier of  FIG. 1  is given as follows. Capacitors C 1  and C 2  (around 0.1 μF or higher) bypass any signal that may cross-feed from grid to cathode or cathode to grid from one tube to another tube. In small signal point of view, the junctions between R 11  and R 12 , and between R 13  and R 14  are shorted to ground by the capacitors C 1  and C 2 . And if resistor value in the order of 1 MΩ or higher is chosen for R 11 , R 12 , R 13  and R 14 , they have insignificant effect to the amplifier in terms of small signal voltage gain and frequency response. 
   On the other hand, capacitor C 3  (around 20 μF or higher) bypasses the resistors R 16 , R 17 , R 18  and R 19 . Therefore, in small signal point of view the resistors R 16 , R 17 , R 18  and R 19  are shorted together. Only resistors R 15  and R 20  remain to function as degenerated resistors as usual. Hence, in the small signal point of view, the differential amplifier of  FIG. 1  functions identically to the conventional one in  FIG. 7  with the same small signal voltage gain and frequency response. 
   On the other hand, and from the DC point of view, the operation of the differential amplifier of  FIG. 1  is given as follows. First of all, it should be noted that when a triode is correctly biased and operates in a steady state, the DC biasing voltage at the cathode is always higher than the DC biasing voltage at the grid. In addition, no grid current flows from grid to cathode. Only DC current flows from the plate to cathode. 
   We assume that the two triodes T 2   a  and T 2   b  are well-matched tubes and the DC biasing voltages passing from the previous stage are also identical. Let us denote the DC potential difference between cathode and grid by V CG , where V CG &gt;0V. Let us also denote the DC biasing current from plate to cathode by I p . In order to minimize mismatch of DC biasing when non-matched triodes T 2   a  and T 2   b  are used, it is best to choose the values for the resistors R 15 -R 20  such that,
 
 R 15 =R 20 ; R 16 =R 19 ; R 17 =R 18  (Eq-1)
 
 R 15 +R 16 =R 17  (Eq-2)
 
 R 19 +R 20 =R 18  (Eq-3)
 
 V   CG   =I   p ·( R 15 +R 16)= I   p   ·R 17 =I   p   ·R 18 =I   p ·( R 19 +R 20)  (Eq-4)
 
   where R 15  and R 20  are the desired degenerated resistors that determine the small signal gain of the differential amplifier. 
   For instance, if V CG =5.5V and I p =6 mA are chosen as the operating DC biasing values for triodes T 2   a  and T 2   b , then the value for R 17  and R 18  can be easily found as 917Ω, or 910Ω, which is the closest practical resistor value. If 100Ω is chosen as the degenerated resistance for R 15  and R 20 , then it can be easily found that R 16  and R 19  is 810Ω, or 820Ω, which is the closest practical resistor value. If the resistors are chosen on the basis of equations Eq-1 to Eq-4, it can be seen in the following that the differential amplifier will have the DC self-biasing ability that minimizes the mismatch due to the triodes T 2   a  and T 2   b , and the mismatch due to the DC biasing voltages passed from the previous stage. 
   We assume now that the two triodes and the DC biasing voltages passed from the previous stage are poorly matched. In such a scenario, when the differential amplifier is powered up, let us denote the DC potential voltages at the grid and the cathode of the tube T 2   a  by V Ga  and V Ca , respectively. Similarly, V Gb  and V Cb  denote, respectively, the DC potential voltages at the grid and cathode of the tube T 2   b . If the tube T 2   a  operates at a higher DC biasing point such that V Ga &gt;V Gb  and V Ca &gt;V Cb , i.e., both grid and cathode DC potential voltages of the tube T 2   a  are greater than tube T 2   b , the series resistors R 11 -R 12  will pass along the higher potential V Ga  and lift up the DC potential at the junction between resistors R 18  and R 19 . As a result, the cathode DC potential (V Cb ) of tube T 2   b  is increased and hence the grid DC potential (V Gb ) is also increased. By the same token, the series resistors R 13 -R 14  will pass along the lower potential V Gb  and bring down the DC potential at the junction between resistors R 16  and R 17 . As a result, the cathode DC potential (V Ca ) of tube T 2   a  is lowered and hence the grid DC potential (V Ga ) is also lowered. Since V Cb  and V Gb  are increased while V Ca  and V Ga  are lowered, V Cb  and V Ca  are pulling closer together and so are the V Gb  and V Ga . Eventually, the differential amplifier of  FIG. 1  will rest on a closer DC biasing point than the one in  FIG. 7 . 
     FIG. 2  reveals a simplified version of  FIG. 1  with no degenerated resistors (i.e. R 15  and R 20  shown in  FIG. 1 ). Since degenerated resistors are not used, the small signal gain of the differential amplifier of  FIG. 2  is higher than the one in  FIG. 1 . For best result, the resistors are chosen such that R 23 =R 24 =R 17 =R 18 . However, without using degenerated resistors to provide local feedback, the amplifier will have higher distortion and lower bandwidth than the one in  FIG. 1 . 
     FIG. 3  shows an alternative circuit arrangement with no bypass capacitor (i.e. C 3  in  FIG. 1  and C 10  in  FIG. 2 ). As no bypass capacitor is used in this arrangement, the resistors R 17 , R 18 , R 23  and R 24  function as degenerated resistors to provide local feedback. Small signal gain is reduced but distortion and bandwidth are improved. For best result, the resistors are chosen such that R 23 =R 24 =R 17 =R 18 . 
   A vacuum tube balanced audio power amplifier employing the new DC self-biased differential amplifier is illustrated in  FIG. 4 . Even without the use of matched vacuum tubes for T 1   a  and T 1   b , T 2   a  and T 2   b , the differential amplifier in the second stage, which has the DC self-biasing ability as described above, will bring the DC biasing point to a closer level compared with the conventional differential amplifier shown in  FIG. 8 . However, there is one scenario in which the vacuum tubes of the differential amplifier in the second stage (i.e., T 2   a  and T 2   b  of  FIG. 4  or T 5   a  and T 5   b  of  FIG. 8 ) will be damaged. 
   Let us assume that in  FIG. 4 , T 1   a  and T 1   b , T 2   a  and T 2   b  are vacuum tubes of different types so that T 2   a  and T 2   b  warm up faster than T 1   a  and T 1   b . When the power amplifier is switched on, all tubes are in cold condition, and therefore they will not draw any plate current. Since there is no voltage drop across the plate resistors R 7  and R 8 , the DC potential at the grid of T 2   a  and T 2   b  is equal to the supply voltage +VS 1 . Also, the cathode of T 2   a  and T 2   b  sit at the supply voltage −VS 5 . Therefore, the grid of T 2   a  is at the DC potential of +VS 1 −(−VS 5 ) above the cathode. It should be noted that at the steady state, the grid potential should be below the cathode potential. But in this cold condition, the polarity is in the opposite. For example, if VS 1 =400V and VS 5 =100V are chosen as the supply voltages, the DC potential of grid-to-cathode when the amplifier is switched on is 500V. If vacuum tubes T 2   a  and T 2   b  get warmed up and start to operate faster than the vacuum tubes T 1   a  and T 1   b , the 500V grid-to-cathode voltage will force grid current to flow and easily damage the tube instantly. Therefore, there is a need to install a protection circuit so as to prevent a large grid-to-cathode voltage from building up when switching on. A power amplifier, which contains the protection circuit, is shown in  FIG. 5 . 
   It can be seen from  FIG. 5  that the protection circuit consists of diodes D 1 -D 2 , zener diodes ZD 1 -ZD 2  and resistor R 35 . The grids of the vacuum tube triodes T 2   a , T 2   b  are connected to the anode of a respective diode D 1 , D 2 , which is respectively connected to the cathode of a zener diode ZD 1 , ZD 2 . The anodes of the zener diodes ZD 1 , ZD 2  are connected with each other, and the resistor R 35  is connected to the junction between the anodes of the two zener diodes ZD 1 , ZD 2  and a constant current source CS 2  which is connected to a negative power supply −VS 5 . 
   The principle of operation of the protection circuit is given as follows. When the power amplifier of  FIG. 5  is switched on, as the vacuum tubes are in cold condition, there is no plate current flow. However, a small current starts to flow immediately from power supply terminal +VS 1  through R 7 , R 9 , D 1 , ZD 1  and R 35  to power supply terminal −VS 5  via current source CS 2 . The grid-to-cathode voltage difference at vacuum tube T 2   a  is now clamped at one diode voltage plus one zener voltage that is much lower than the +500V potential difference. Similarly, a small current also starts to flow immediately from power supply terminal +VS 1  through R 8 , R 10 , D 2 , DZ 2  and R 35  to power supply terminal −VS 5  via current source CS 2 . Again, the grid-to-cathode voltage difference at vacuum tube T 2   b  is clamped at one diode voltage plus one zener voltage. Therefore, the circuit effectively protects the tubes by avoiding a large grid-to-cathode voltage to build up when switching on. 
   When the tubes get warmed up and start to operate, plate currents begin to flow. If the zener diode is properly chosen, the diode and zener will be eventually turned off. In order to ensure that the diode and zener diode work properly, we should choose the diode and zener such that: 
   Diode forward voltage+zener reverse voltage&gt;voltage drop of grid-to-cathode (V GC ) of vacuum tube T 2   a  (or T 2   b )+voltage drops across resistors R 15 , R 16  and R 17  (or R 18 , R 19  and R 20 ). 
   The above condition will hold true as long as there is no input signal. To prevent the diode and zener from turning on in the steady state when a signal passes through the grid, we should choose the zener reverse voltage such that:
         Diode forward voltage+zener reverse voltage&gt;voltage drop of grid-to-cathode (VGC) of vacuum tube T 2   a  (or T 2   b )+voltage drops across resistors R 15 , R 16  and R 17  (or R 18 , R 19  and R 20 )+maximum signal&#39;s voltage swing at the grid of the vacuum tube T 2   a  (or T 2   b ).       

   For instance, if we follow the above same example, we have the following: 
   a) V GC =−V CG =−5.5V; 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   Let us assume that the maximum signal voltage swing at the grid of the vacuum tube=5V. 
   If we take 0.7V as the diode forward voltage, the zener diode reverse voltage is found to be 9.78V or higher. R 35  is a small value resistor that can be ignored in the above calculation. If we choose a 15V zener diode for the above application, the amplifier works in the desired manner such that the zener diodes are turned on to protect the vacuum tubes when the vacuum tubes are in cold condition. The zener diodes are then turned off during the steady state, when the vacuum tubes are in normal operation, and they do not affect the signals being amplified. 
     FIG. 6  shows the complete differential amplifier that has the DC self-biasing ability and grid-to-cathode over-voltage protection. 
   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 that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any appropriate sub-combinations.