BEAM CURRENT MEASUREMENT IN A VIDEO OUTPUT STAGE

Abstract: The invention discloses a beam current measurement device with a new manner of subtracting the quiescent current through the emitter-follower transistors from the total current through these emitter-follower transistors to obtain a measurement of the actual beam current. Instead of two current mirrors, two folded cascode circuits are used. The measured beam current will then be the actual current plus an offset caused by the difference between the currents from the two current sources used with the folded cascodes.

Description: BEAM CURRENT MEASUREMENT IN A VIDEO OUTPUT STAGE The invention relates to a beam current measurement device, and to a picture display apparatus comprising such a beam current measurement device. US-A-4,680,640 (PHN 11,505) discloses a picture display device in which errors occurring in the beam current measurement in a video output stage are reduced by increasing the recharging rate of the capacitive load (constituted by the picture display tube) of a video amplifier with the aid of a bias current or quiescent current which is subtracted again from the current applied to an input of the beam current measuring circuit which is controlled by the video amplifier. More specifically, a cathode of the picture display tube is coupled to the emitter of an emitter-follower drivable by the video amplifier. A negative feedback input of the video amplifier is coupled to an input of the emitter-follower. The collector of the emitter-follower is coupled to an input of the beam current measuring circuit. An anti-blocking circuit is coupled to the cau ode of the picture display tube to prevent blocking of the beam current measuring circuit due to leakage currents in the picture display tube. The anti-blocking circuit produces an emitter-follower current of the order of magnitude of at least approximately 100 μA. A current source is coupled to the input of the beam current measuring circuit for draining the emitter-follower current produced by the anti-blocking circuit. In one embodiment, the emitter-follower forms part of a complementary emitter-follower circuit having an additional emitter-follower which is complementary to the first-mentioned emitter-follower, the anti-blocking circuit generates a bias voltage for the basis of the emitter-followers, and the current source is a current mirror circuit an input of which is coupled to an output of a further current mirror circuit an input of which is coupled to the cathode of the additional emitter-follower. In this embodiment, it appeared that if the actual transfer functions of the current mirrors deviate from their respective nominal transfer functions such that the actual transfer functions are a and β times the respective nominal transfer functions, the part of the actual cathode current which flows thru the current mirrors is affected by the ratio of these deviations α and β, and the measured cathode current is formed by the sum of the part of the actual cathode current which does not flow thru the current mirrors and which is thus unaffected, the part of the actual cathode current which flows thru the current mirrors and which is affected, and the bias current thru the emitter-follower transistors multiplied by a factor depending on a and β. The bias current thru the emitter-followers depends on the manner in which the output amplifier is controlled, and may vary to a large extend and even become zero. This means that in certain circumstances, the distortion in the measured cathode current which is caused by the deviations in the transfer functions may reach values of the order of magnitude of the actual beam current, so that the beam current measurement is seriously affected. It is, inter alia, an object of the invention to provide an improved beam current measurement. To this end, a first aspect of the invention provides a device as defined in claim 1. A second aspect of the invention provides a picture display apparatus as defined in claim 5. Advantageous embodiments are defined in the dependent claims. A beam current measurement device according to a primary aspect of the invention, comprises an output transistor for supplying a cathode current, a measurement output for supplying a measured current representing me cathode current, a first current source coupled to a first main terminal of the output transistor for supplying a first current, and a second current source coupled to a second main terminal of the output transistor for supplying a second current substantially equal to the first current, in which a first cascode transistor is coupled between the first main terminal of the ouφut transistor and the measurement output. A primary embodiment of the invention provides a new manner of subtracting the quiescent current thru the emitter-follower transistors from the total current thru these emitter-follower transistors in order to obtain a measurement of the actual beam current. Instead of two current mirrors, two folded cascode circuits are used. As will be set out below, the measured beam current will then be the actual beam current plus an offset caused by the difference between the currents from the two current sources used with the folded cascodes. The advantage of this circuit is that these current sources provide constant currents which cause a constant offset independent from the quiescent currents in the emitter- follower. Further advantages of a preferred embodiment of the invention comprising the folded cascodes are: - The output range of the emitter-follower ouφut stage is not limited. - The measuring circuit input is not directly connected to the emitter-follower, so that the voltage at the measuring circuit input does not influence the ouφut range of the ouφut stage. - The maximum current to the measuring circuit input is limited to the current from the current sources of the folded cascodes. Especially with fast amplifiers or power amplifiers, the currents in the ouφut stage can reach very high values. In such a case, it is very desirable to have these currents flow locally instead of thru the long loop of the long loop of the measuring circuit as in the prior art. - Except for the offset current, the cathode current is applied to the measuring circuit input without distortion. The invention renders it possible to measure small (dark) beam currents in the order of magnitude of 10 μA in a very accurate manner. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a first embodiment of a picture display apparatus in accordance with the invention; Fig. 2 shows part of a second embodiment of a beam current measuring device in accordance with the invention; Fig. 3 shows part of a third embodiment of a beam current measuring device in accordance with the invention; Fig. 4 shows part of a fourth embodiment of a beam current measuring device in accordance with the invention; and Fig. 5 shows part of a fifth embodiment of a beam current measuring device in accordance with the invention. In the embodiment of Fig. 1 , a bias voltage source Vbias is connected between the gates of first and second field-effect transistors Tl , T2 which are connected in a complementary "emitter-follower" (source-follower) configuration. The bias voltage source Vbias may be formed with diodes as in Fig. 2 of US-A-4,680,640 (PHN 11,505), with two bipolar transistors having their respective bases connected to their respective collectors, or with two MOSFETs having their respective gates connected to their respective drains. The sources of the transistors Tl, T2 are connected to the cathode C of a display tube D. The drain of the transistor Tl is connected to a 200 V supply line thru a bias current source Ibiasl, and to a measuring ouφut OUTM thru a transistor Ta. A bias voltage Vbias 1 is applied to the gate of the transistor Ta. The drain of the transistor T2 is connected to ground thru a bias current source Ibias2, and to the measuring ouφut OUTM thru a transistor Tb. A bias voltage Vbias2 is applied to the gate of the transistor T . The measuring ouφut OUTM is connected to a beam current measuring circuit (not shown). Zener diodes D21 and D22 are connected in parallel to the bias current sources Ibiasl and Ibias2, respectively, to handle large currents flowing thru the source-follower transistors Tl, T2 during normal operation. During die black current measurement to which the invention pertains, the currents thru the source-follower transistors Tl, T2 are smaller than the bias currents Ibiasl and Ibias2. For the remainder of the picture display apparatus, reference is made to Fig. 2 of US-A- 4,680,640 (PHN 11,505), herein incorporated by reference. The cathode current lc is split up into a current lea thru transistor Tl and a current Icb thru transistor T2, which currents lea, Icb are superposed on die bias current lb thru transistors Tl, T2 caused by the bias voltage Vbias applied to their gates. In contrast to me prior art in which one of me parts lea, Icb of the cathode current lc is conveyed to the measuring ouφut tiiru two current mirrors, while me other part is directly conveyed to me measuring ouφut, now both parts lea, Icb of the cathode current lc are conveyed to the measuring ouφut OUTM thru a folded cascode stage. When the actual cathode current lc is zero, current Id thru transistor Ta equals Ibiasl - lb, current Ie mru transistor Tb equals Ibias2 - lb, and current Im supplied to the measuring ouφut OUTM equals Id - Ie which equals zero in the ideal situation in which Ibiasl = Ibias2. In a less ideal situation, mere is a small deviation between Ibiasl and Ibias2, such that Ibiasl = α.Ibias2, which results in that Im = (α-l).Ibias2. Consequently, even with non- ideally equal bias currents Ibiasl , Ibias2, the measuring current Im does not depend on the bias current lb mru me emitter-follower transistors Tl, T2. When lc is taken into account, Id = Ibiasl - (Ib-Ica), Ie = Ibias2 - (lb + Icb), and Im = Id - Ie = lea + Icb + Ibiasl - Ibias2 = lc + (c.-l).Ibias2. Consequently, the measured camode current Im is formed by the actual cathode current lc plus a substantially constant and small offset current. The embodiment of Fig. 1 can be simplified by omitting the second cascode transistor Tb, so that the measuring ouφut OUTM is directly coupled to the second bias current source Ibias2. While mis modification is simpler and cheaper, the second bias current Ibias2 is less easily kept constant as the voltage over the second bias current source Ibias2 is no longer constant. In another modification (not shown) of the embodiment of Fig. 1 , both me source-follower transistor T2 and the second cascode transistor Tb are omitted, where the cathode C of the image display tube D and the source of the transistor Tl are directly coupled to the second bias current source Ibias2, while the drain of the cascode transistor Ta and the measuring ouφut OUTM are coupled to a further bias current source. In this modification, it holds that Ibiasl = Ibias2 + a further bias current supplied by the further bias current source. As a consequence of me non-zero ouφut impedances of the current sources Ibiasl , Ibias2, these bias currents depend to some extent on the bias current thru the emitter-follower transistors. By actively keeping the source voltages of the cascode transistors Ta, Tb constant, so mat the voltages over the current sources Ibiasl , Ibias2 are kept constant, and by varying the gate voltages, it is possible to substantially completely suppress the dependence of me bias currents Ibiasl, Ibias2 on the bias current lb thru the emitter- follower transistors. It should be noted that Im is limited by Ibiasl and Ibias2, and that both Ibiasl and Ibias2 should exceed lb. Fig. 2 shows part of a second embodiment of a beam current measuring device in accordance with the invention, in which this improvement is implemented. Only the upper part of the circuit is shown; the lower part is easily derived from the shown upper part. The bias current Ibias 1 is provided by an ouφut of a current mirror T3, T4. A current source Iba is connected to a drain of a cascode transistor T5. A current source Ibb is connected to a drain of a transistor T6 coupled for supplying a bias current to transistor T5. The source of transistor T6 connected to the input of the current mirror T3, T4. The source of transistor T5 is connected to me drain of a transistor T7 having a source which is connected to the 200 V supply line, and a gate which is connected to the ouφut of the current mirror T3, T4. The drain of the transistor T5 is also connected to the gate of a transistor T8 having a source which is connected to the ouφut of me current mirror T3, and a drain which supplies me current Id. So, transistor T8 in Fig. 2 corresponds to transistor Ta in Fig. 1. Transistor T8 is the cascode transistor which is controlled by transistor T7 via transistor T5. Transistor T7 ensures that the voltage over transistor T3 remains constant, such that me gate-source voltage of transistor T7 causes a drain current Iba thru transistor T7. Transistor T5 is a cascode transistor for transistor T7 so that the voltage over transistor T7 remains constant even if the current lb varies, and consequently, the control voltage of the transistor T8 also varies (as a current thru a transistor can only vary if the control voltage of the transistor varies). W(Tx) is herewith defined to indicate the width of the channel of the MOS field-effect transistor Tx. By choosing the ratios Iba/Ibb, W(T7)/W(T4) and W(T5)/W(T6) to be equal to each other, where the length of me channel of T7 equals the lengm of me channel of T4, and the length of me channel of T5 equals me length of the channel of T6, it is ensured mat the voltage over transistor T3 is identical to the voltage over transistor T4, so that a perfect current mirroring is obtained. The previous embodiments show me following advantages: - the maximum and minimum ouφut voltages of die emitter-followers are not limited by the measuring circuit. - a very high accuracy is possible. - the cathode current lc appears substantially undistorted in the measured current Im. - the measurement is not disturbed by the quiescent current lb. - no high peak currents are supplied to me measuring ouφut OUTM. - a very high ouφut impedance is present at the measuring ouφut OUTM. In a more refined embodiment as shown in Fig. 3, the cascoding of the transistor T7 with transistor T5 can also be made active. The accuracy of Ibiasl is only dependent on the voltage variation at the drain of transistor T5, and thus by the ouφut impedance of transistor T5. By actively controlling me gate of transistor T5, it is possible to increase the ouφut impedance of transistor T5. Fig. 3 differs from Fig. 2 in that transistor T6 is no longer present, in that the input of the current mirror T3, T4 is directly connected to the current source Ibb, and by providing an additional transistor Tc having a source connected to the 200 V supply line, a gate connected to the junction point of the drain of transistor T7 and the source of transistor T5, and a drain connected to a current source Ibc and the gate of transistor T5. Fig. 4 shows an even better implementation, in which transistor T6 is reinserted between me input of me current mirror T3, T4 and the current source Ibb. A transistor Td is inserted, having a source connected to the drain of the transistor Tc, a gate connected to the gate and the drain of transistor T6, and a drain connected to the gate of transistor T5 and the current source Ibc. It will be evident mat similar alterations can be effected, for example, by rendering Td active in a similar manner as T5 has been made active. Thus will result in increasingly smaller transistors and quiescent currents. Fig. 5 shows part of a fifth embodiment of a beam current measuring device in accordance with me invention. The embodiment of Fig. 5 is an alternative to the embodiment of Fig. 2. A single "emitter-follower" transistor Ti l has a source connected to the cathode of a picture display tube (not shown), a gate connected to an ouφut amplifier stage (not shown), while a speed-up capacitor Cs is connected between the gate and me source of the transistor Ti l. The source of the transistor Ti l is coupled to a 200 V supply line mru a bias current source Ibiasl 1. A drain of the transistor Ti l is connected to an ouφut of a current mirror T13, T14, and to ground mru a zener diode Zl. The input of me current mirror T13, T14 is connected to me source of a transistor T16. A gate and drain of the transistor T16 are connected to a 12 V supply line thru a current source Ib2. A source of a transistor T15 is connected to a drain of a transistor T17. A source of the transistor T17 is connected to ground. A gate of me transistor T17 is connected to the junction point of the zener diode Zl and the ouφut of the current mirror T13, T14. Transistor T15 is a cascode transistor for transistor T17 and transistor T16 supplies a bias current for transistor T15. A drain of me transistor T15 is connected to me 12 V supply line thru a current source Ibl, and to a gate of a transistor T18. A source of the transistor T18 is connected to the junction point of the zener diode Zl and the ouφut of the current mirror T13, T14. A drain of me transistor T18 is connected to the measuring ouφut OUTM and to the 200 V supply line thru a current source Ibiasl2. A current Ibiasl3 thru the ouφut transistor T13 should be equal to the sum of the currents Ibiasl 1 and Ibiasl2. The zener diode Zl clamps the current Ibiasl3 during transients in which me cathode current exceeds the currents Ibiasl 1 and Ibiasl2. Similar clamps are also added in the other embodiments. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and mat those skilled in me art will be able to design many alternative embodiments without departing from the scope of the appended claims. While field-effect transistors are shown, bipolar transistors are also possible but field-effect transistors render a very high accuracy possible in view of the absence of gate currents. In the claims, any reference signs placed between parenmeses shall not be construed as limiting the claim.

Claims: Claims: 1. A beam current measurement device, comprising: an ouφut stage (T1,T2; Ti l) for supplying a cathode current (lc); a measurement ouφut (OUTM) for supplying a measured current (Im) representing the camode current (lc); means (Ibiasl; T13) for supplying a first current (Ibiasl; Ibiasl 3), the first current supplying means (Ibiasl; T13) being coupled to a first main terminal of the ouφut stage (T1 ,T2; Ti l); means (Ibias2; Ibiasl l,Ibiasl2) for supplying a second current (Ibias2; Ibiasl 1+ Ibiasl 2) substantially equal to said first current (Ibiasl; Ibiasl3), the second current supplying means (Ibias2; Ibiasl l,Ibiasl2) being coupled to a second main terminal of the ouφut stage (Tl ,T2; Tl 1); and a first cascode transistor (Ta; T18) coupled between the first main terminal of the ouφut stage (T1 ,T2; Ti l) and the measurement ouφut (OUTM). 2. A beam current measurement device as claimed in claim 1, further comprising a second cascode transistor (Tb) coupled between me second main terminal of the ouφut stage (T1,T2) and me measurement ouφut (OUTM). 3. A beam current measurement device as claimed in claim 1, wherein the first current supplying means (Ibiasl) comprise: a current mirror (T3, T4) having an ouφut coupled to said first main terminal of the ouφut stage (T1.T2); and a cascode arrangement (T5, T6, T7) including a cascoded transistor (T7) for keeping a voltage at said first main terminal of the ouφut stage (T1,T2) constant, a cascode transistor (T5) having a first main terminal coupled to said cascoded transistor (T7) and a second main terminal coupled to a first auxiliary current source (Iba) and to a control terminal of said first cascode transistor (T8), and a control transistor (T6) having a first main terminal coupled to an input of said current mirror (T3, T4) and a second main terminal coupled to a second auxiliary current source (Ibb) and to control terminals of said cascode transistor (T5) and said control transistor (T6). 4. A beam current measurement device as claimed in claim 1 , wherein the first current supplying means (Ibiasl) comprise: a current mirror (T3, T4) having an ouφut coupled to said first main terminal of the ouφut stage (T1,T2); a first cascode arrangement (T5, T7) including a primary cascoded transistor (T7) for keeping a voltage at said first main terminal of the ouφut stage (T1 ,T2) constant, and a primary cascode transistor (T5) having a first main terminal coupled to said cascoded transistor (T7) and a second main terminal coupled to a first auxiliary current source (Iba) and to a control terminal of said first cascode transistor (T8); and a second cascode arrangement (Tc, Td, T6) including a secondary cascoded transistor (Tc) for keeping a voltage at said first main terminal of the primary cascode transistor (T5) constant, and a secondary cascode transistor (Td) having a first main terminal coupled to said secondary cascoded transistor (Tc) and a second main terminal coupled to a second auxiliary current source (Ibc) and to a control terminal of said primary cascode transistor (T5), and a control transistor (T6) having a first main terminal coupled to an input of said current mirror (T3, T4) and a second main terminal coupled to a third auxiliary current source (Ibb) and to control terminals of said secondary cascode transistor (Td) and said control transistor (T6). 5. A picture display apparatus, comprising: a picture display tube (D) having a camode (C) to which a cathode current (lc) is applied; and a video signal ouφut stage coupled to the cathode (C), the video ouφut stage including a beam current measurement device for measuring the cathode current (lc) as claimed in claim 1.

Inventors: VAN DER ZEE PIETER, VAN DER ZEE, PIETER
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N5
IPC codes: H04N5