DE2509981A1 - GAIN CONTROL CIRCUIT IN A HIGH FREQUENCY CIRCUIT - Google Patents

Description

Description of the patent application

the company Indesit Industria Elettrodomestici S.p.A., Rivaita (Torino / Italy), Strada Piossasco Km 17

concerning:

"Gain Control Circuit in a High Frequency Circuit"

The invention relates to a gain control Circuit in a high frequency circuit that has a transmission stage for high frequency signals. In particular, the invention relates to a circuit for the Control of the gain of a color subcarrier frequency signal in a television receiver. This gain control can be necessary, for example to manually adjust the color saturation or, in the case of automatic gain control, the output color signal level independently of To keep changes in the input signal amplitude due to changes in propagation or drift in the characteristic values of certain Receiver elements are based. Numerous gain control circuits are known; in the case of high frequency signals, for example of color subcarrier frequency signals in a color television receiver, it is common to use amplifiers with damped resonance circuits. In this case, at

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known gain control circuits introduced significant phase rotations into the processed signal. These known circuits are based, for example, on changing the operating point of an emitter-base circuit operated TRansistor in order to control its base current and thus its collector current in such a way that it is increased (direct forward control) or relatively reduced (reverse control) corresponds to the value of the maximum gain. However, when the operating point is changed, this changes considerably the transconductance and input resistance of the transistor and phase rotations are inevitable. In on a typical circuit, the rotation can be as much as 18o.

It is also known to use a common collector transistor for the gain control circuit and its working point, i.e. the emitter current, to change. Apart from the fact that you are with one Circuit of this type cannot achieve a voltage gain, the input impedance changes considerably in function of reinforcement. Accordingly, phase rotation is inevitable.

Another known circuit is in a publication by R.C. Thieling and A.V. Korolenko in the I.E.E.E. Transactions described using the so-called Ebers-Moll effect. This circuit, according to the explanations in the cited publication pronounced for the automatic gain control of color subcarrier frequency signals was developed in a television receiver, uses an amplifying transistor in emitter-base circuit with a negative feedback resistor across the emitter. This resistance is parallel to the collector-emitter path a second transistor connected to the emitter of the first via

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a capacitor is coupled. By changing the base current of the second transistor one achieves one Change in impedance and accordingly a change in gain of the first transistor. Even in this case, however there is a certain phase shift because it affects the input impedance of the first transistor is affected by the impedance changes across the emitter. In the mentioned publication the problem of phase rotation is not discussed; it literally means:

¹¹ In some previous attempts to develop DC voltage controlled amplification, a field effect transistor was used. This was an ^a measured choice / since using an FET as a variable resistor is a well known use case. This circuit was otherwise similar to the one used later. It worked, but the linearity was poor and the signal processing capacity was unsatisfactory. "

However, the phase shift is undesirable in a color television receiver. · In two of the systems that are used for transmission are used by color television signals, the color saturation depends noticeably on the phase of the subcarrier (NTSC and PAL system).

Usually one tries the adverse influence on the image due to phase shift caused by gain control by the following measures avoid:

by letting the automatic gain control act on the chrominance signal that is still both contains the reference signal as well as the color signals; this is an attempt at the relative phase shift between the former signal and the latter signals

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to be eliminated, but it is evident that some difference error remains inevitable;

by providing manual saturation control, which attenuates the signal directly (mechanical resistance attenuators); this has the disadvantage that the color signals have to travel a long way within the device and the combined saturation and Contrast control becomes complicated and expensive;

by using the phase error compensation properties a PAL circuit having a delay line; in this case, however, it results in addition to the considerably higher costs for the recipient, the disadvantage of a remaining saturation error (Differential gain error).

The object of the present invention is to create a circuit with which the disadvantages of the known circuits be avoided in order to achieve a gain adjustment, especially for color subcarrier frequency signals in color television receivers, which circuit should be simple and in particular no noticeable phase shift should introduce.

This task is performed in a gain control circuit in a high frequency circuit with a transmission stage for the high-frequency signals with at least one first semiconductor element with more than two electrodes, of which at least one is a signal input electrode, one is a signal output electrode, and one is a common input and output is associated electrode and having at least one second element which is substantially like a

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variable resistance behaves according to the invention in that at least a first resistor and the second element connected in series between the common electrode of the first semiconductor element and a reference potential is, and that the gain of the first semiconductor element is controlled by significantly changing the Equivalence resistance of the second element without any significant shift in the working point of the first element, so as to ensure that the gain change is accompanied by a noticeable phase shift in the signals.

An exemplary embodiment of the invention is explained in more detail below with reference to the attached documentation, showing the circuit diagram of a chrominance signal amplifier for color subcarrier frequencies with automatic gain control for a color television receiver embodying the principles of the present invention.

With 1 is a complete chrominance signal with subcarrier frequency symbolizes that contains the color signals and the reference signal (burst); it is assumed that this signal is derived in a known manner from composite video signals, which in turn from the received television signal was derived. The reference number 2 symbolizes an input terminal of an amplifier circuit with automatic Gain control to which the signal is applied. A coupling capacitor 3 is connected to the terminal 2 and is located with its other end to a capacitor 4, a resistor 5 and an inductor 6, the other terminals of which, respectively are connected to ground and thus form a damped parallel resonance circuit that is tuned to the subcarrier frequency is. The hot end of this damped parallel resonance circuit is connected to a terminal 7, on which tapped the reference signals (burst signals) by means of known circuits (not shown) of the television receiver will. This node is also connected

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- ο -

with a decoupling resistor 8 in series with a capacitor 9, although the direct current component blocks, but couples the signal to the downstream amplifier, namely to the Baäs of an NPN transistor 22 in emitter-base circuit, which represents the regulated gain meter. The reference symbol Io symbolizes a pulse signal at line frequency, which is simply the familiar line retrace pulse with negative polarity can act. It is on the terminal 11, which is connected to a coupling capacitor 12 in series with a resistor 13, the other end of which is connected to the cathode of a Zener diode 14, whose The anode is connected to ground and its Zener voltage is much smaller than the peak-to-peak amplitude of the pulse Io (at least by half).

The cathode of the Zener diode 14 is connected to a decoupling resistor 15, the other terminal of which is connected to the base of the transistor 22 »with reference numeral 16 marks a power supply source to which a positive voltage + V is permanently applied, which is significantly higher than the Zener voltage of the diode 14. This terminal 16 is connected to one end of a resistor 17, the other End to ground is via a capacitor 18, with the together it forms a decoupling filter, and the same thing The end of the resistor 17 is also connected in parallel via a capacitor 19 to a resistor 2o and an inductance 21 connected to the collector of transistor 22. Capacitor 19, resistor 2o and inductance 21 together form a damped parallel resonance circuit that is tuned to the subcarrier frequency. The emitter The transistor 22 is connected to a network formed from two capacitors 23 and 24 and a resistor 25 in parallel connection. The other end of this network is

- 7th

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connected to a resistor 26 which is connected to the drain electrode of an N-type field effect transistor 27 is grounded with its source electrode and connected to a resistor with its gate electrode 28. This resistor 28 is connected to the collector of a second NPN transistor 29. A resistor 3o is placed between the collector of transistor 29 and ground.

The emitter of the transistor 29 is connected to the junction of a fixed resistor 31 and a variable resistor

32 connected as a voltage divider between a terminal

33 and ground are laid. A permanent negative supply voltage -V is connected to terminal 33. A capacitor

34 is placed between terminal 33 and the base of transistor 29; this base is also connected to the cathode a diode 35. The Anoder of the diode 35 is connected to a resistor 36, the other end of which is connected to the Terminal 33 is connected to a capacitor 37.

Terminal 38 is the output terminal for the amplified Gain controlled signal, and a capacitor 39 is between the Kdlektor of the transistor 22 and the terminal 38. A capacitor 4o and two resistors 41 and 42 are in series between the collector of transistor 22 and ground. The common point of these two resistances is connected to the BasLs of an NPN transistor 43. A resistor 44 is between the terminal 16 and the base of the Transistor 43. The emitter of transistor 43 is connected to ground via a resistor 45 in series with the parallel circuit a resistor 46 and a capacitor. The collector of the transistor 43 is connected to this Capacitor 37 and also with one end of a third parallel resonance circuit, which is tuned to the frequency of the subcarrier and consists of a capacitor 48, a

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Resistor 49 and an inductance 5o. The other end of this third resonance circuit is from the Terminal 16 fed via a decoupling filter, consisting of a resistor 51 in series and a bypass capacitor 52> which is connected to ground. The way of working of this automatic gain control circuit is as follows:

The complete chrominance signal 1 arrives at the base of the Amplification transistor 22 through capacitor 3 and resistor 8 in series with capacitor 9 and is filtered through the resonance circuit, consisting of capacitor 4, resistor 5 and inductance 6. The transistor 22 is alternately switched through and blocked by the impulses Io, which are made into a rectangular shape and are clamped by the Zener diode 14. More precisely, the transistor 22 is blocked during the flyback time and is turned on for the remainder of the sampling period. It works so that it has the reference signals (Burst) transmits or suppresses while the color signals (information signals) are allowed through. The amplified signal at the collector of the transistor 22, filtered by the resonance circuit consisting of the capacitor 19, resistance 2o and inductance 21, is connected to terminal 38 for supplying the synchronous demodulators but also at the base of the transistor 43. The transistor 43 amplifies the chrominance signal further, which then is filtered by the circuit, consisting of capacitor 48, resistor 49 and inductance 5o, and the signal lies at the collector of this transistor 43. From here it reaches the demodulation diode 35 via the capacitor 37. When the DC voltage obtained in this way, which is a function of the amplitude of the chrominance signal, exceeds the threshold exceeds that is determined by the voltage divider

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from the resistors 31 and 32, it controls the conductivity of the transistor 29 and consequently the voltage across the collector resistance of transistor 29. This voltage is eem gate of the field effect transistor 27 supplied and changes its equivalent resistance. There the transistor 27 is in the emitter lead of the transistor 22, changes the resistance change of this resistance the negative feedback effect and thus the gain of the transistor 22. At the same time changes the emitter current of transistor 22 also by a certain amount. By suitable selection of the values of the Resistors 25 and 26, however, it is possible that this Stromändcung is within limits at which the Parameters of the transistor 22 are not noticeably influenced, in particular its input impedance. To this In this way, an undesired phase shift of the amplified color signal at the terminal 38 is avoided. The emitter current of transistor 22 is given in this regard with good nutrition by the equation:

VV
I _E = Z ^V BE

R25 + R26 + R27

where V _{17 is the} voltage of the zener diode 14, V _n - the base-emitter forward voltage of the transistor 22, R25, R26 and R27 are the values of the resistors 25 and 26 and the equivalent resistance of the transistor 27. The change in emitter current is given by the equation:

I "= R25 + R26 + R27 max
R25 + R26 + R27 min

where R27 min is the equivalent minimum resistance of the transistor and R27 max is the equivalent maximum resistance of the same transistor. The voltage gain of the Transistor 22 is given when it is sufficiently large

- Io -

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- Io -

Current gain (h__) by the approximate equation:

t egg

G = R2o

R26 + R27

where R2o is the equivalent resistance of the resonance circuit in the collector lead, which is approximately equal to the value of the resistor 2o, if the quality factor of the circuit is sufficiently high. The gain change is therefore given by the equation:

r = ^R26 + R27 max
R26 + R27 min

This equation can be written in the form:

t ?? 7 - R26 (GI) + G
^R27 - R27 min

wherein:

R27 max

R27 =

R27 min

Similarly, Equation 2 can be written, in the form:

- (825 + R26) ( ¹ E -1) + ¹ E R27 min

By equating 5 and 7 one obtains the equation: G (R26 + R27 min) = I "(R25 + R26 + R27 min) - R 25,

£ 1

which is the change in gain dear G and the change in current I “relates to each other.

It can be seen that with the circuits described it is possible within certain limits, I "small to be made against G. If namely R25 is large relative to R26 + R27 min, one can write 8:

G = (I-1) R25

R26 + R27 min

which means that if R25 is large relative to R26 + R27 min, G is large relative to I ". This possibility R25 great

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- Ii -

to make / is only limited by the voltage + V that is available and the capacity of transistor 22 to withstand a high voltage + V. For a gain control of 34 dB, however, corresponding to a ratio of 1 to 5o, with R27 ⁱⁿ loo ohms and R27 max = Io Kohm from equation 4:

c _ R26 + loooo loooo

^bO "~ R26 + loo R 26 + loo

from which one obtains:

R26 = loo ohms.

By using the same values and limiting the change in current to a value of 2, one obtains from Equation 9:

R25 = 5ox2oo = Ιο, οοο Ohm.

If a minimum current of 1 mA is required, one obtains from 1:

V ₁₇ = 1.lo " ³ χ loooo + V ^ _x , = lo.6 volts

Δ Dili

and therefore a voltage + V in the order of magnitude of 15-2o volts is sufficient, which is absolutely normal.

It is obvious that if transistor .22 does not have to work as a gate at the same time, it is possible to feed it with a substantially constant base current so as to ensure that the emitter current is complete becomes independent of the gain. In the specific case of the circuit described, it was not necessary to achieve such a high degree of reinforcement ratio and the values used, which are only given here as an example, were the following:

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3 capacitor 27o pF 4th capacitor 68o pF 5 resistance 47o ohms 8th resistance 1.8 Kohm 9 capacitor 47o pF 12 = capacitor o.l, uF 13 = resistance 3.3 Kohm 14 = - diode ZW 3 15 = resistance 1 Kohm 17 = resistance loo ohms 18 = capacitor loo, uF 19 = capacitor 68o pF 2ο = resistance 33o ohms 22 = transistor BF 196 23 = capacitor 22 nF 24 = capacitor 22, uF 25 = resistance 68o ohms 26 = resistance 12 ohms 27 = transistor 2N 3822 28 = resistance 22o Kohm 29 = transistor BC 148 3ο = resistance 12 Kohm 31 = resistance 1 Kohm 32 = Potentiometer loo Kohm 34 = capacitor 4.7 nF 35 = diode BA 129 36 = resistance 3.9 Kohm 41 = resistance 1 Kohm 42 = resistance 15 Kohm 43 = transistor BF 197 44 = resistance 22 Kohm 45 = resistance Io ohm 46 = resistance 47o ohms 47 = capacitor Io, uF 48 = capacitor 68o pF 49 = resistance 82o ohms 51 = i & nieSSii-ir loo ohms 52 = capacitor 22, uF

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37 = capacitor 1 nF

39 = capacitor Io nF + V = 12 volts

40 = capacitor 1 nF - V = 12 volts

As mentioned, the circuit described is an amplifier for chrominance signals at color subcarrier frequency and with automatically regulated gain for use in a color television receiver of the PAL type, to keep the amplitude of the chrominance signal constant with changing saturation and with changing type of the transmitted scene. It is obvious that for this Use case is significant that the regulated level introduces no rotation in the phase of the chrominance signal, since such rotation in a receiver of this type results in a color density change between even and odd numbers Lines leads. The circuit described is capable of a gain control of more than 2o dB without To bring about the introduction of a noticeable phase shift. Circuits according to prior techniques for such a gain ratio introduced a rotation of the order of 3o-4o degrees, but because this is quite unacceptable is simple in a receiver of the PAL type, which, however, is expediently avoided even in a receiver of the PAL de luxe type with a delay line (which, as mentioned, transforms phase errors into saturation errors, accordingly the cosine of the phase shift angle) and the final saturation for rotations of the order of magnitude mentioned in an order of magnitude of 5-15%.

In addition to its application in automatic gain control for chrominance signals of the type described, the variable gain gain circuit may fail of phase rotation according to the invention can also be used to advantage in automatic gain control circuits

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for complete chrominance signals (color signals with reference signals) or in circuits for manual Saturation adjustment; at present there is a tendency to use this setting more and more To effect DC voltages so that you can combine them with the contrast setting and so you can use the control button Can accommodate in a position far away from the actual circuit without the risk dangerous couplings. This applies to both PAL receivers with a delay line as well as in still larger dimensions on simple PAL or NTSC receivers. The benefits of variable gain circuits without Phase rotation according to the invention is evident from the above explanations. You can also see that numerous modifications are possible without departing from the basic principle of the invention.

(Patent claims)

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Claims (19)

Claims Gain control circuit in a high frequency circuit with a transmission stage for high-frequency signals with at least one Semiconductor element with more than two electrodes, of which at least one signal input electrode a second is a signal output electrode and a common input electrode and output is associated with and having at least one second element that is substantially behaves as a variable resistor, characterized in that at least one first resistor and the second element are connected in series between the common electrode of the first Semiconductor element and a reference potential and that the gain of the first semiconductor element is controlled by significantly changing the Equivalence resistance of the second element without any significant shift in the operating point of the first element, so as to ensure that the gain change does not change from a noticeable Phase rotation of the signals is accompanied. 2. amplification control circuit according to claim 1, characterized in that that a damped resonance circuit is connected to the output electrode of the first semiconductor element is. 3. Gain control circuit according to any one of the preceding Claims, characterized in that the input electrode of the first semiconductor element a damped resonance circuit is connected. 509837/0700 - 16 - 4. Gain control circuit according to one of the preceding claims, characterized in that that the signal transmission stage is part of an automatic gain control loop is. 5. Gain control circuit according to one or more of the preceding claims, characterized characterized in that the first semiconductor element is a transistor. 6. gain control circuit according to claim 5, characterized in that the signal input electrode of the transistor whose base terminal is, the signal output electrode is the collector and the common electrode of the emitter. 7. gain control circuit according to one of the preceding claims, characterized in that the second element is a semiconductor element with more than two electrodes, of which at least one is a control electrode, one is an output electrode and one is a common electrode, the Input and output is assigned, and that the second semiconductor element with its output or common electrode is connected to the first resistor or the reference potential. 8. gain control circuit according to claim 7, characterized in that the second semiconductor element is a field effect transistor. 9. gain control circuit according to claim 8, characterized in that the control electrode of the second semiconductor element is the gate electrode, the output electrode is the drain electrode and the 509837/0700 - 17 - common electrode the source electrode of the field effect transistor. 10. Gain control circuit according to an or several of the preceding claims, characterized in that the first resistor or at least one capacitor connected in parallel to at least one part of the first resistor is. 11. Gain control circuit according to one or more of the preceding claims / characterized characterized in that the first resistor has a considerably greater ohmic resistance as the equivalent minimum resistance of the second element. 12. Amplification control circuit according to claim 11, characterized in / that the first resistor has an ohmic resistance about 100 times greater than the equivalent minimum resistance of the second resistance. 13. gain control circuit according to one or more of claims 4-12, characterized in that that at least part of the signal from the output of the transmission stage to an amplitude detector is fed. 14. gain control circuit according to claim 13, characterized in that the detector is a threshold detector is. 15. gain control circuit according to claims 13 or 14, characterized in that a continuous voltage at the output of the detector is used for the 509837/0700 • / if. Control of the resistance of the second element. 16. Gain star circuit after one or more of the preceding claims, characterized in that the circuit is part of a television receiver. 17.. A gain control circuit as claimed in claim 16, characterized characterized in that the high frequency signals are signals with color subcarrier frequency. 18. gain control circuit according to claim 17, characterized in that the signal transmission stage at the same time works as a gate circuit. 19. Gain control circuit according to claim 17, characterized in that the signal transmission stage can be used for manual adjustment of color saturation. 509837/0700