Stefano Perugini for PaengDesign
Copyrigth 2001
0) Introduction
Back in early years of XX century the first thermionic vacuum tube appeared as a
two-electrode device (named Vacuum Diode) and was used mainly in the rectification process of alternating current for power supplies and as detector element for the early Amplitude Modulation radios.
In the 1907 the introduction of a third electrode (called Grid) with the function of
control of the plate current, by DeForest, it will transform the Thermionic Vacuum Diode into a new device named Triode.
The Triode allowed the development of electronic amplifiers and later the born of the Switching Electronic (i.e. Digital Electronic). After the Triode, the insertion of more electrodes will generates other devices classes like Tetrodes, Pentodes and Hexodes.
Solid State electronic, starting from semiconductor diodes and continuing by the
subsequent introduction of Bipolar Junction Transistor (BJT), Junction Field Effect Transistor (JFET), MOSFET, BiCMOS etc. have progressively undermined the domain of Vacuum Tubes until to the practical disappearance.
The major reason for the displacement of Thermionic Vacuum Tubes basically
during the decade 1955-1965 by Solid State microelectronic devices was the following:
1) The need to heat the cathode surface in order to extract the electrons required to establish a current into the vacuum: a heating process requiring inevitable waste of electric power;
2) A poor current density from the material necessary to build cathodes;
3) A high value for the operating voltages (in the order of hundred and often thousand of volts) and a low current capability (i.e. the device has an intrinsically high internal resistance);
4) A fabrication process very critical from a mechanic point of view and three-dimensional occupation never less than one-cubic centimeter.
5) The drift of electrical parameters reverberated from the good(bad)ness of themechanical assembly.
Despite this, today Thermionic Devices retain much selected niches in the field of Microwave Power Electronics, Electro Medicals, High Energy Physics, Professional and Consumer Audio markets. Quite surprisingly thermionic vacuum tubes are re-gaining popularity and a significant re-using for Audio Application. In fact often, Musicians, Technicians and Audio Engineers seem to prefer equipments fitted with these “glowing” devices.
Despite the intrinsic nature of high voltage/ low current device, vacuum tubes have an unsurpassed intrinsic linearity very suitable for the building of electrically simple preamplifiers, power amplifiers, Digital to Analog Converter output stages, Mixers,
Sound Effect gears and so on with low rate of total stages and (in the case of
Amplifiers) low rate of applied negative feedback with subsequent very low value of Intermodulation Distortions.
Sector studies confirm that both professionals and consumers involved in the Audio often prefer goodness equipped with vacuum tube devices.
Nevertheless the above mentioned limitation of vacuum tubes still remains as primal obstacle to a full renaissance and a deeper penetration in the above mentioned market
segments.
This article it describes a Bipolar Junction Transistor – Vacuum Tube connection able to emulate the Langmuir Child’s Law which rules the physics of thermionic vacuum tube. The suggested connection(s) it will permit to use vacuum tubes in the region of Low Voltages and high current in order to embrace the powerfulness of solid state devices and retaining the advantages offered by tubes.
A further advantage is that despite the gained current capability the proposed connection doesn’t need more wasted power for the thermionic heater.
1) The Triode
The Triode is a thermionic vacuum tube with three active elements. Fig. 1 shows the circuital symbol with the meaningful electrical parameters
From circuit symbol of Fig. 1 it has been removed the indications for the heater element used in the practical device to establish on the Cathode’s surface the thermionic emission. The active elements are the plate (P), the grid (G) and the cathode (K). IP is the plate current, Vg is the grid voltage and VP is the plate voltage. Normal operation mode requires that plate-to-cathode voltage VP be positive and the grid-to-cathode voltage Vg be negative. When these conditions are
meet the triode obey to the Langmuir-Child’s law (alternatively named 3/2 power
law):
where KT and are constants inherently determinated by electrical/mechanical characteristic of the particular thermionic triode.
The constant is named amplification factor and represents the ratio of the incremental change in the plate voltage VP to the incremental change in the control grid VG under the condition that the plate current IP remains unchanged. There are two other principal triode coefficients named transconductance (gm) and plate resistance (rp). The transconductance gm is the incremental change in the plate current IP divided by the incremental change in the grid voltage VG under the condition that the plate voltage VP remains unchanged. The plate resistance rp is the incremental change in the plate voltage VP divided by the incremental change in the plate current IP under the condition that the grid voltage VG remains unchanged. The relationship between gm and rp under the condition that all have been measured on the same operating point is:
Since vacuum tubes are device with very high rp from eq.[2] it descends that gm is very low (in the order of few mA/V) and this it reflects on small current capabilities for the device also with very powered cathode heaters. This limitation can be surpassed with very high plate voltages if the goal is to obtain meaningful output power from the device.
2) Triode Plate Characteristics
Usually the electrical characteristics of thermionic vacuum tubes are set down in the form of graphical curves. The graphical curves represent an useful way to record plate currents when specified voltage on grid and plate are applied. Curves are published by the tube manufacturer and represent the averaged behavior of the particular tube. The manufacturer set also the so-called electrical tolerances reflecting the drift from the averaged tube due to the assembling. A very important class of graphical curves are the plate characteristics. In order to draw this curves grid voltage VG is kept constant, the
plate voltage VP is changed step-by-step and the plate current IP is recorded at each step of plate voltage VP. The procedure is repeated for a further increment on the grid voltage
VG. At he end of this process it is possible to reproduce a family of plate curves as shown in Fig.(2).
Two main considerations must be pointed-out by the observation of Fig. 2:
a) Triode plate currents are intrinsically low (in the order of units of mA or at most tens of mA for power triodes used in the field of telecommunications) despite an heating power of units of watt for low power triodes and tens of watt for power triodes.
b) Plate voltages are usually in the order of hundreds of volt for signal triodes and thousands of volt very common for power triodes.
Again these are the consequence of the inherent nature of thermionic vacuum tube: a device with elevated plate resistance, low transconductance and poor cathode current density also with high heating power. It must be underlined that high plate voltages VP are not an absolute requirement. Triodes can operate also with low plate voltages (in the order of tens of volt) but the associated plate current are negligible if the grid voltage is not placed close to zero; nevertheless when this happens very complex physical phenomena undermine the linearity of the device.
3) The Bipolar Junction Transistor (BJT)
The familiarity with solid state devices it permits to report only few meaningful considerations worth for the proceeding this article about the Bipolar Junction Transistor (BJT).
The Fig. 3 shows the circuital symbol of a BJT. The active elements are the collector (C), the base (B) and the emitter (E). IC, IB and IE are the collector current, the base current and the emitter current respectively. Under normal operating conditions the collector-to-emitter voltage VCE is positive and the base-to-emitter voltage VBE is biased about at 0.6V.
In these conditions
Where is a constant named BJT’s current amplification. Since IC >> IB, it is possible to neglect IB from the first equation in [3] and obtain
Basically, from eq.[4] it results that in the above mentioned operating condition the BJT
acts like a simple base current IB .
4) The Bipolar to Vacuum Tube Triode Connection (BiVTriode)
In this chapter it will be introduced a BJT-to-Vacuum Triode connection (BiVTriode for short) in order to overcome the limitations shown in the chapter 2) about the thermionic vacuum triode. By combining in a specific manner a vacuum triode with a BJT it is possible to obtain a kind of “supertriode” with the following remarks:
i) Low plate operating voltages;
ii) High plate currents (i.e. low internal resistance)
iii) Intrinsically high electrical linearity
Fig. 4 shows the BiVTriode connection. By observing the Fig. 4 the following main consideration must be underlined:
a) K=B (the triode cathode is electrically connected to the BJT base); b) P=C (the triode plate is electrically connected to the BJT collector); c) and since IK=IP):
[5] IP= IB
By combining eq(s): 1), 4) and 5) the following expression can be written:
The eq. [6] can represent the Langmuir-Child’s Law for the BiVTriode.
Just like a vacuum triode, the electrical behavior of a BiVTriode can be summarized by the plotting the family of curves derived from eq.[6] following the same procedure illustrated in the chapter 2. Further, according to eq.[6] the family of plate curves for a BiVTriode
are a very good replica of Fig2 but having the following superior advantages:
i) Higher emitted currents IE (just like a BJT)
ii) Lower plate operating voltage VCE? VPE (just like a BJT)
iii) High intrinsic linearity (just like a triode vacuum tube)
Fig. 6 shows a proposed symbol for the generic BiVTriode connection. The active elements are the plate (P) or the Collector (C), since P?C, the grid (G) and the emitter (E). From a practical perspective a BiVTriode emulates the behavior of a vacuum triode in the
region of high currents and low voltages just like a BJT with the electrical behavior of a standard vacuum triode. Just like a thermionic device is a voltage amplifier device and the BJT is a current amplifier device, a BivTriode can be seen as transconductance amplifier since it transforms the voltage VG into the current IE.
All of these characteristics are very appealing for the replacement of Vacuum
triodes with BiVTriodes in the circuitries commonly found on all-tube equipments.
5) The BiVTriode in Diode mode (BiVDiode)
A vacuum diode behavior in the region of high current/low voltage region typical of a solid state diode can be obtained with the simple arrangement derived from Fig.4 and shown in Fig. 7.
From Fig. 7 it descends:
The Langmuir-Child’s Law for the device shown in Fig. 7 can be derived from eq. [6]. Since G? P eq.[6] becomes
Eq. [7] represents the Langmuir-Child’s law of the BivDiode shown in Fig. 7.
The plate current characteristic for a generic BivDiode is shown in Fig. 8.
From Fig. 8 it descends that a BiVDiode will follow the Langmuir Child’s law in the region of High Currents/Low Voltages after the BJT threshold of 0.6V.
Fig. 9 shows a proposed symbol for the generic BiVDiode connection. The active elements are the plate (P) or the Collector (C), since P?C?G and the emitter (E). From a practical perspective a BiVDiode emulates the behavior of a vacuum diode in the region of high currents and low voltages just like a solid state counterpart but with the electrical behavior of a standard vacuum diode that’s retaining the physical behavior summarized by the eq. (7).
6) The Bipolar to Vacuum Tube Pentode Connection (BiVPentode)
The concepts illustrated on chapters (4) and (5) can be expanded to multigrid thermionic devices as tetrodes (2 control grids ) and pentodes (2 control grids, 1 suppression grid).
Here will be examinated the derivation of a BivPentode from a generic vacuum pentode but similar considerations can be extended to BivTetrodes (BJT and Tetrode connection) and other multigrid vacuum devices as Hexodes.
The pentode is a vacuum tube with five active elements.
Fig. 10 shows its circuit symbol. Again, and for simplicity, the heater element for establish the thermionic emission is not represented in Fig. 10.
The active elements are the plate (P), the control grid (G), the screen grid (S), the
suppressor grid and the cathode (K). The device operates with the suppressor grid electrically connected to cathode (K) as shown in Fig. 10.
Normal operation mode requires that the plate-to-cathode and the screen grid to- cathode voltages be positive and control grid-to-cathode be positive.
When this condition are met the device obeys to Langmuir-Child’s Law in the
form
where KP is a constant, c is the control grid amplification factor and s is is the screen grid amplification factor.
By using the same procedure used to derive the family of triode plate curve, here
we can plot a family of pentode plate curves. Usually pentode plate curves are bi- dimensional plotted for a fixed VS.
Example of family plate curves is shown in Fig. 11.
Similar considerations thereto of triode must be pointed-out by the observation of Fig. 11:
a) Pentode plate currents are intrinsically low (in the order of units of mA or at most
tens of mA for power pentodes used in the field of telecommunications, although higher than triodes) despite an heating power of units of watt for low power pentodes and tens of watt for power ones.
b) Plate voltages are usually in the order of hundreds of volt for signal pentodes and thousands of volt very common for power ones.
c) The presence of a screen grid positively biased it shape the curves like a solid
State device with a linear and saturation region.
7) The Bipolar to Vacuum Tube Pentode Connection (BiVPentode)
In this chapter it will be introduced a BJT-to-Vacuum Pentode connection (BiVPentode for short) in order to overcome the limitations shown in the chapter 6) about the thermionic vacuum pentode. By combining in a specific manner a vacuum pentode with a BJT with a similar connection introduced for BiVTriodes it is possible to obtain a kind of “superpentode” with the following remarks:
i) Low plate operating voltages;
ii) High plate currents.
Fig. 12 shows the BiVPentode connection. By observing the Fig. 12 we can reply the following main considerations borrowed from the BivTriode connection:
d) K=B (the pentode cathode is electrically connected to the BJT base); e) P=C (the pentode plate is electrically connected to the BJT collector); f) and since IK=IP (pentode cathode current equal to bjt base current):
[9] IP= IB
By combining eq(s): 4), 8) and 9) the following expression can be written:
The eq. [10] represents the Langmuir-Child’s Law for the BiVPentode.
Just like a vacuum pentode the electrical behavior of a BiVPentode can be summarized the plotting the family of curved derived from eq.[10] following the same procedure illustrated in the chapter 6. Further, according to eq.[10] the family of plate curves for a BiVTriode are a very good replica of Fig. 11 but having the following superior advantages (Fig. 13):
i) Higher emitted currents IE (just like a BJT);
ii) Lower plate operating voltage VCE? VPE (just like a BJT).
Fig. 14 shows a proposed symbol for the generic BiVPentode connection. The active elements are the plate (P) or the Collector (C), since P?C, the control grid (G), the screen grid (S) and the emitter (E). From a practical perspective a BiVPentode emulates the behavior of a vacuum pentode in the region of high currents and low voltages just like a BJT but with the electrical behavior of a standard vacuum triode.
Just like a thermionic device is a voltage amplifier device and the BJT is a current amplifier device, a BivTriode can be also seen as transconductance amplifier since it transform the voltage VG into the current IE.
All of these characteristics are very appealing for the replacement of Vacuum
pentodes with BiVPentodes in the circuitries commonly found on all-tube equipments.
By adding further connection on the device in Fig. 14, it is possible to obtain from a BivPentode (a) a PseudoBivTriode (b) and a PseudoBivDiode (c) as shown in Fig. 15.
8) The Spice Model for the generic BivTriode
In this chapter will we introduce Spice Models for the BivTubes (BivPentode, BivTriode, BivDiode) introduced in the previous chapters in order to evaluate the goodness of the proposed connections.
Firstly, we’ll focus on a BivTriode since to generate a BivDiode model it’s sufficient to encode into Spice code the connection depicted in Fig. 7 starting from the model of the BivTriode. For this purpose we’ll use an ad-hoc combination of Spice
model related to two commercially available devices as the 6922 small vacuum tube (Fig.
16) and ZTX690 low power high BJT. Obviously many other combinations can be used for this purpose.
Table 1 shows a possible Spice Model for a generic BivTriode by combining the two above mentioned commercially available devices
Fig. 17 shows a comparison between stand alone 6922 and the BivTriode of Table 1 in terms of plotted plate curves derived from a Spice voltages sweep.
Fig. 17 is a comparison between plate curves of the 6922 thermionic vacuum tube SPICE model with the generic SPICE model of the BivTriode (combo of 6922 Vacuum tubes + ZTX690B BJT). Plate voltage sweep is 0V to 100V. Grid voltage sweep is 0V to -3V with step of -0.5V. The electrical characteristics we can extract from examine of Fig.18 for a common operating point of VP=30V, VG=-0.5V are summarized in Table 2.
The examen of results written in Fig. 18 and Table 2 it results that the BivTriode acts as a sort of super 6922 Vacuum Tube having the same amplification factor but with a
much higher transconductance gm and a much lower rP. From a practical point of view the BivTriode connection of Fig.4 reduces the plate voltage operating range from hundred of Volts to tens of volt and improving dramatically plate current delivery as shown by the comparison of transconductance and plate resistance.
9) The Spice Model for the generic BivDiode
The simple connection of Fig. 7 it transforms a BivTriode into a BivDiode that’s as a device behaving like a thermionic vacuum diode. To generate a SPICE model for the generic BiVDiode we can start from Table 1 and collapse node 1 and 2. The resulting SPICE models is shown in Table 3.
The Table 4 shows the SPICE model of the commercially available Thermionic
Vacuum Diode EZ81 (Fig. 18).
The Fig. 19 summarizes the plate curves both for the EZ81 and the generic BivDiode.
The BivDiode (excluding a threshold voltage of about 0.6V imposed by the base-emitter diode junction) shows a greater current delivery respect to EZ81 for a same plate-to- cathode voltage drop. Further, since thermionic vacuum diodes have big dimension and dissipate huge amount of cathode heating power (in the order of tens of watt), the BivDiode represents a very appealing replacement , in fact for a same plate-to-cathode voltage drop of 2V we obtain the results summarized in Table 5.
10) The Spice Model for the generic BivPentode
In this Chapter we reproduce the same procedure of the last two chapter comparing SPICE model of a generic BivPentode and the standard, commercially available Thermionic Pentode 6AP3C. By referring to the connection of Fig. 12 we can build the SPICE model for a generic BivPentode. The model merge SPICE model of the 6AP3C pentode with the ZTX690B BJT, Table 6.
Fig. 21 are the related plate curves for the generic BivPentode and the stand-alone
6AP3C.
Also in this case we can re-propose the same summarizing consideration of the last two chapters.
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