Grid prestress
The low DC voltage of the control grid of an amplifier stage built up with a tube, which is negative compared to the cathode , is called grid bias . The grid bias defines the operating point of a tube circuit with other parameters .
The field effect transistor is also a voltage-controlled quadrupole , for which the same applies.
purpose
If you want to control an electron tube without power, no grid current must flow despite the superimposed control voltage . For this purpose - in order not to load the signal source - a small negative voltage is supplied via a very high resistance (approx. 1 MΩ), the value of which determines the operating point. This bias voltage should be chosen so that the grid voltage (in relation to the cathode voltage) never becomes positive even at the maximum amplitude of the superimposed signal voltage, so that no undesired rectifier effect occurs, which usually generates distortions .
For preamplifiers, one always selects the operating point A , which lies roughly in the middle of the approximately linear range of the characteristic. This means that the distortions are minimal even without negative feedback . This is shown in the adjacent picture:
- With a grid bias voltage of −0.5 V ( A operation), the sinusoidal change in the anode current (top right) corresponds very precisely to the sinusoidal curve of the grid voltage. This is called linear, undistorted amplification.
- With a grid bias voltage of −2 V ( AB operation), the anode current is no longer sinusoidal, i.e. it is distorted.
For push-pull power amplifiers in the LF range, the operating points AB and B are preferred because, due to the higher grid bias, the quiescent current and thus the power loss of each tube is lower than in A mode. As a result of the greater curvature of the characteristic there is always noticeable distortion, which is either desired ( guitar amplifier ) or reduced by negative feedback.
For high-frequency power amplifiers, the operating point C is often chosen with such a negative grid bias that it would be around −30 V in the characteristic curve shown. If the superimposed signal voltage is so high that the grid voltage is more positive than the cathode voltage in the amplitude peaks, the tube serves as an on / off switch for the anode current with particularly low power loss and an efficiency of over 75%. The resulting enormous distortion must be eliminated with a subsequent selective filter .
Source of grid tension
Separate voltage source
The clearest solution is a separate voltage source, which is provided either by a battery or by the power supply unit.
The positive connection of the separate voltage source is usually connected to the circuit ground. The negative connection is connected to the grid leakage resistor in the case of a capacitor coupling or to the winding of the transformer in the case of a transformer coupling. In the case of directly heated tubes, the connection to the negative connection of the heating battery may be sufficient.
With direct voltage coupling, the separate voltage source is usually located in the signal path between the amplifier stages and can be designed as a Zener diode .
The automatic grid bias generation
The quiescent current flows through the resistor R 2 , at which the bias voltage ΔU (z. B. 2 V) arises. The cathode is therefore at a higher potential (voltage level) than the control grid, which is at ground potential via the very high resistance R 1 . If the resistor R 2 should not change the dynamic properties, that is to say the AC voltage amplification of the tube, it is bridged with a capacitor C 2 of sufficient capacity. The time constant of this cathode combination influences the lower limit frequency of the circuit. For direct current , R 2 acts like a negative current feedback that stabilizes the operating point. This principle of stabilizing the operating point is also used in transistor circuits.
No current flows in the control grid circuit, so there is practically no voltage drop across this high-resistance grid leakage resistor and the grid bias occurs as a potential difference between the control grid and cathode with the correct sign: it makes no difference whether the control grid is more negative than the cathode or the cathode is more positive than the control grid .
The automatic grid bias generation cannot be used if the mean anode current fluctuates strongly as in B operation, because the operating point then shifts depending on the signal. It is only suitable for signals free of DC components (symmetrical to the zero line), for example for amplifiers in A mode .
The semi-automatic grid tension generation
Some receiver circuits use a composite tube with a common cathode connection for two tube systems (ECL11, PCL81). Because each of these systems needs a different grid bias, this grid bias cannot be generated with a single cathode combination - there is only one cathode and thus only one cathode resistor.
As a solution, in practice the negative lead of the power supply unit is not connected directly to the circuit ground but via a resistor . Due to the total current of the supplied circuit, a voltage drop of the necessary level of the grid bias occurs at this resistor. The control grid of the composite tube is connected to the actual negative line of the power supply unit via a high-resistance grid leakage resistor and a filter element: Since there is no significant current flowing through the control grid, the bias voltage is applied directly to the control grid.
This resistor can be built up as a voltage divider from different resistors or consist of a (wire) resistor with extra taps (clamps) on which different grid pre-voltages can be set and removed for different tubes. Since the output tube usually requires the highest value of the grid bias voltage of a circuit, the resistance is measured accordingly, while the grid of the preamplifier tube located in the same bulb receives only part of the negative bias voltage. Regardless of the type of end tube and its grid bias generation, a different type of bias generation can be used in other stages. The grid bias of the VHF pre-stage can be generated automatically, for example, while that of the VHF mixer is generated by grid rectification (audion effect).
Because not every tube generates its own grid bias, but the grid bias is influenced by the cathode current of all tubes, one speaks of semi-automatic grid bias.
It should be noted that a defective tube in a circuit can affect the grid voltage of all other tubes that are fed by semi-automatic generation.
In some data sheets for this type of composite tube, it is therefore stated how large the percentage of the cathode currents of the composite tube in the total current requirement of the circuit must be in order to obtain adequate operating point stabilization, as is the case with automatic grid bias generation.
The clamping
The not unproblematic clamping is used advantageously for signals that contain very low frequencies (fractions of Hertz). Since the bias voltage arises at least partially on the clamping capacitor, the addition of signal and bias voltage is particularly clear here.
Both the signal and the reference value (clamping potential) must be provided with a low internal resistance during clamping so that the clamping can move the operating point quickly enough. As a rule, in such cases, the terminal potential or the reference value in the signal curve is viewed as the operating point. This working point is then asymmetrical in the dynamic range and z. B. in picture tubes at the anode current starting point.
The grid starting current
The principle of clamping is used for capacitor coupling in input stages whose modulation is low. In the case of low-frequency pre-stages, a low grid bias is often sufficient, which occurs as a voltage drop across a relatively large grid leakage resistor. The typical value of the resistance is 10 MΩ.
The bias voltage is generated in the tube system itself by electrons that land on the control grid (instead of flying through it) and thus shift its potential towards the negative. The particularly high- resistance leakage resistance ensures a certain flow of current and thus a stabilization of the process so that the grid does not “clog” itself.
The grid rectification
In audion , the DC grid voltage is created by rectifying the signal at the grid current start point . The changing DC voltage is the demodulated signal. This effect of the circuit can also be interpreted as clamping the maxima of the signal to the grid current application point.
The circuit principle of grid rectification is also used in oscillator circuits to generate the grid bias. Because the term audion is primarily associated with demodulation, this type of generating the grid bias is often described as “according to the type of audion” ( Tropadyne ).
Grid leakage resistance
Despite the normally powerless control, the control grid must have a certain potential. In the simplest case, it is connected to ground potential with a resistor. This resistance is called the grid leakage resistance. It derives the relatively small number of electrons from the grid which, due to their inertia, hit the grid electrode despite the negative grid voltage (also known as the starting current).
The resistance must be as high as possible in order not to reduce the input resistance of the stage unnecessarily (100 kΩ ... 1 MΩ). If it is chosen to have a sufficiently high resistance (10 MΩ), a noticeable voltage drops across it and the grid bias voltage is generated by the starting current.
In oscillator circuits , the grid leakage resistance is always chosen to be relatively small (approx. 30 kΩ) in order to prevent the oscillator from working unintentionally as a blocking oscillator at significantly lower frequencies (see superregenerative receiver ).
The grid leakage resistor does not necessarily put the control grid on ground potential; it can also be applied to another, defined DC voltage potential in order to achieve the desired operating point setting.
See also
literature
- Heinrich Barkhausen: Electron tubes . 4th edition. Published by S. Hirzel, Leipzig 1937.
- Friedrich Benz: Introduction to radio technology . 4th edition. Springer-Verlag, Vienna (1937, 1950, 1959).