Cathode bias

As bias is defined as the low - opposite the cathode - a negative DC voltage of the control grid of a built-up with a tube amplifier stage. The bias sets the operating point of a tube circuit with other parameters.

The field effect transistor is also a voltage-controlled quadrupole, applies to the analog.

The purpose of the grid bias

If you want to control an electron tube without power, no grid current is allowed to flow in spite of superimposed control voltage. For this purpose - not to load the signal source - a very high resistance (about 1 M ) supplied to a small negative voltage, the value of which determines the operating point. This bias voltage is to be chosen so that even at maximum the amplitude of the superimposed signal voltage, the gate voltage ( with respect to the cathode voltage ) is not positive, so that no undesirable rectifier effect occurs, which is usually generated distortion.

For preamp always chosen the operating point A, which is located approximately in the middle of the substantially linear portion of the characteristic curve. This distortion is minimal, even without feedback. This is shown in the picture:

• At a bias of -0.5 V (A mode), the sinusoidal variation of the anode current (top right) corresponds quite exactly to the sinusoidal variation of the grid voltage. This is called a linear, undistorted amplification.
• At a bias of -2V ( AB operation ) of the anode current is not sinusoidal, thus distorted.

For push-pull power amplifier in the LF range, the operating points AB and B are preferred because due to the higher bias voltage of the bias current and thus the power loss of each tube is less than the A- mode. Due to the greater curvature of the curve, it always comes to noticeable distortions that are either desirable ( guitar amp ) or be reduced by negative feedback.

For high -frequency power amplifier is often the operating point C is chosen as the negative bias that this would be in the characteristics shown for example at -30 volts. In addition, when the higher-level signal voltage is so large that the amplitude peaks of the grid voltage is more positive than the cathode voltage, the tube serving as a power switch for the anode current with a very low power loss and an efficiency over 75%. The resulting massive distortions must be removed with a subsequent selective filter.

Source of bias

Separate voltage source

The clearest solution is a separate power source, provided either from a battery or from the AC adapter.

The positive terminal of the separate power source is usually connected to the circuit ground. The negative terminal is connected with capacitor coupling with the Gitterableitwiderstand or transformer coupling to the coil of the transformer. In directly heated tubes connect to the negative terminal of the heating coil may be sufficient.

When DC coupling the separate power source is generally in the signal path between the amplifier stages, and may be embodied as a zener diode.

The automatic Gittervorspannungserzeugung

The quiescent current flowing through the resistor R2 at which the bias DELTA.U ( 2V, for example ) is formed. Therefore, the cathode is at a higher potential ( voltage level ) and the control grid, which is located at the very high impedance Massepotetial via resistor R1. If the resistance R2, the dynamic properties, ie the AC gain of the tube should not change, it is bypassed with a capacitor C2 sufficient capacity. The time constant of this combination of the cathode influences the lower limit frequency of the circuit. R2 works for DC as a current feedback that stabilizes the operating point. This principle of stabilization of the operating point is also used in transistor circuitry.

The control grid circuit, no current flows, so formed in this high resistance Gitterableitwiderstand virtually no voltage drop and bias occurs when the potential difference between the control grid and cathode with the correct sign on: whether the control grid relative to the cathode is negative, or the cathode in opposition to the control grid positive makes no difference.

The automatic Gittervorspannungserzeugung can not be applied if the average anode current varies strongly as in the B- mode, because then shifts the operating point dependent on the signal. She is (symmetrical to the zero line ) Only suitable for equal share free signals, such as amplifiers in class A mode.

The semi-automatic Gittervorspannungserzeugung

Some receiver circuits use a composite tube with a common cathode connection for two pipe systems ( ECL11, PCL81 ). Because each of these systems needs a different bias, this bias can not be generated with a single Katodenkombination - there are only a cathode, and so only a cathode resistor.

As a solution to the negative lead of the power supply is connected not directly but through a resistor to the circuit ground in practice. By the total current supplied to the circuit formed in this resistor, a voltage drop in the necessary amount of bias. The control grid of the composite tube is placed through a high Gitterableitwiderstand and a filter section to the actual negative lead of the power supply: Since no appreciable current flows through the control grid, the bias voltage is applied directly to the control grid.

This resistance can be built up as a voltage divider from different resistors or consist of a ( wire ) resistance with extra taps ( clamps ), where you can set different for different grid bias tubes and remove. Since the end tube usually requires the highest value of the bias voltage of a circuit, the resistance is then measured while the preamp present in the same flask lattice gets supplied to only a part of the negative bias voltage. Regardless of the type of output tube and their Gittervorspannungserzeugung can be applied in other stages a different kind of bias voltage. The grid bias of the FM precursor can be generated for example automatically, while the FM mixer stage by lattice rectification arises ( Audion effect).

Because not every tube generates its own bias, but the bias is influenced by the cathode current for all tubes, it is called semi-automatic grid bias.

Note that a broken tube may affect the gate voltage of all the other, fed with semi-automatic production of tubes in a circuit.

In some data sheets for this type composite tubes is given about how large the percentage of the cathode currents of the composite tube must be the total power requirements of the circuit to obtain an adequate operating point stabilization as in automatic Gittervorspannungserzeugung.

The clamping

The clamping is not unproblematic advantageously used for signals which contain very low frequencies ( fractions of a Hertz). Since the bias voltage is at least partially formed on the clamping capacitor, in this case, the addition of signal and bias voltage is particularly pronounced.

Both the signal and the reference value ( terminal potential ) must be provided in the clamp with a low internal resistance, so that the clamping can shift the operating point quickly enough. Usually the terminal potential and the reference value is regarded as an operating Sigalverlauf in such cases. This operating point is then asymmetrically in the dynamic range and, for example in picture tubes on the anode current starting point.

The starting current grids

The principle of clamping is applied to capacitor coupling in input stages, the modulation is low. At low frequency is often sufficient precursors low bias which arises as voltage drop across a relatively large Gitterableitwiderstand. The typical value of the resistance is 10 M.

The bias is self- generated in the tubes by electrons landing on the control grid (rather than fly through ) and thus shift its potential for negative way. The particularly high resistance bleeder resistor provides a certain current flow and thus a stabilization of the process, so that the grid does not even " plug up ".

The lattice rectification

When the grid Audion DC generated by the rectification of the signal at the grid current starting point. The changing DC voltage is the demodulated signal. This effect of the circuit can also be interpreted as a clamping of the maxima of the signal to the grid current starting point.

The circuit principle of the grid rectifier is also used in oscillator circuits to produce the bias. Because the term Audion mainly demodulation is connected, this way of generating the bias often associated with " the nature of the Audions " circumscribed ( Tropadyne ).

Gitterableitwiderstand

Despite usually loose power control, the control grid must have a certain potential. In the simplest case, one applies it to a resistor at ground potential. This resistance is referred to as Gitterableitwiderstand. He derives the relatively small number of electrons from the lattice, which strike the grid electrode because of their inertia despite negative grid voltage ( also referred to as start-up current ).

The resistance should be as high impedance to avoid unnecessarily reduce the input resistance of the stage ( 100 k ... 1 M ). If it is chosen to be sufficiently high impedance ( 10 M ), falls on him from a significant voltage, it comes to Gittervorspannungserzeugung by starting current.

In oscillator circuits of Gitterableitwiderstand is always chosen to be relatively small ( approximately 30 kOhm ) to avoid that the oscillator operates as a blocking oscillator unintentionally at considerably lower frequencies (see the super-regenerative receiver).

The Gitterableitwiderstand puts the control grid is not necessarily at ground potential; it can also be placed on a different, defined DC potential to provide the desired working point.