Negative resistance components like tunnel or ESAKI diodes [4.040]
offer an alternative possibility of realizing multi vibrator and hence also discriminator circuits. Because of,their short response times in the sub-nanosecond range, the tunnel diode discriminators are used espe-cially in fast circuits.
Fig. 4.30 shows the basic circuit diagram of a tunnel diode (TD) multivibrator and its corresponding characteristics. The load resistor R and the unavoidable parasitic capacity C of the diode and of the related circuitry are connected in parallel to TD. The current I from a current source is divided between the diode (Io) and the other two components (Ie,IR ). On the one hand, 10 depends on VD in conformity with the diode characteristic ID= f(Vo) exhibiting a negative resistance Rn volt-age range, on the other hand, the load R defines a load line 10= I - V o/R.
Of the three points of intersection of these two curves, (assuming of course
IR I> IRn I)
only the points (A) and (B) are stable. In these oper-ating points I = I R+
10 holds and Ie = O. If the input current I of the circuit exceeds the value 11 , the state (A) disappears; if I is decreasedunder 12 , the state (B) disappears. The plot of VD (which is at the same time the output voltage of the circuit) versus the input current I ex-hibits a shape characteristic of multivibrators (cf. Fig. 4.02) with a hysteresis In = 11 - 12, During the. transition, a current Ie = I - I R - I D
is available for recharging the capacity C. As can be seen from the characteristics, the transition 11 is better defined and faster than the transition 12 , The smaller the difference between
IRI
andIRnl,
the smaller is the hysteresis In.Fig. 4.30. Basic tunnel diode circuit
The tunnel diode TD in parallel with the resistor R can also be considered as a positive feedback amplifier with the (voltage) loop gain bA = JRJ/JR.J. The positive feed-back is realized in a particularly simple manner, since the tunnel diode is an active dipole and the output as well as the input lie at the same terminal. The considerations stated in Chapter 4.11 concerning the magnitude of the hysteresis and the transition times as functions of the loop gain bA apply accordingly.
As can be seen from Fig. 4.30, a TO multivibrator, like the Schmitt trigger, can be biased outside of the hysteresis range (i.e. 1<12 or 1>11)' In this case the operating range is limited only by the maximum ratings of the diode current. More sensitive discriminators are obtained when monostable or bistable circuits biased between 11 and 12 are used. Of course, bistable circuits must be reset by some external means [4.041,
4.049]. Two variants of a monostable multivibrator are shown in Fig.
4.31. A small resistor R
URJ
< JRn\) is used, thus giving only one inter-section (A) of the static load line with the diode characteristic. The operating point (A) is adjusted by means of the standing current 10 ,t-+--r-o Vout
'0
R
a b c
Fig. 4.31 a-c. Tunnel diode monostable multivibrator with coil L (a) or cable (b), and the related characteristics
In the circuit a) a coil with inductivity L ~
JRnJ'
tt is connected in series with R. Here tt denotes the transition time of the circuit. Hence, at least during the transition, the coil L represents an infinite resistance and the dynamic load line "L" becomes horizontal. Any input current pulse lin raising the dynamic load line over the tunnel diode peak cur-rent Ip causes the circuit to flip over into the state (B') and later (B).However, the point (B) is not stable and the circuit returns to the initial state (A) as soon as L passes enough current to reduce 10 below the valley current Iv' The resulting pulse length is approximately equal to L/(JRJ
+
JRn\) [4.042]'In the circuit b) a time-dependent load is obtained by connecting in series a resistor R and a coaxial cable with the characteristic im-pedance Z = R. Obviously the circuit operates if R <
JRnJ
< 2R holds.The pulse length is given by twice the delay time of the cable.
For a more detailed discussion of the tunnel diode discriminator, some considerations concerning the transition time tt are necessary. In Fig. 4.32 a the basic circuit diagram is shown once more with the stand-ing current generator 10 and a load R = 00 (i. e. IRis negligibly small).
The input current pulse has an amplitude lin and a length bin' The dif-ference Uo+lin)-Ip is denoted by ,Min' Obviously only .1Iin >O trig-gers the circuit. Due to C the transition occurs only as fast as the
para-sitic capacity is recharged. In Fig. 4.32 the current Ie available for charging C is plotted separately as a function of YD' The curve is passed in the direction indicated by arrows, and the corresponding time scale t(VD) can be obtained by integrating Cj1c:
Vo
a
ID
b
T-lin
d
1_~t---~-+'~\I
'D(4.21)
Fig. 4.32 a -d. Switching characteristics of a TD trigger
Fig. 4.33 a shows some pulse shapes for <>in = 00 (i. e. current step at the input) with Lllin as parameter. The small current Ie in the output volt-age range Vo ... VI yields a very slow pulse beginning. Hence in practice the pulse appears delayed by a time tOl defined by VD(tOl) = VI' The exact value of tOl , of course, depends on the actual shape of the tunnel diode characteristic. If 10 lies in close proximity to Ip' Ie(VD) can be approximated by a parabola in the range Vo ... VI' with corresponding tOl of
(4.22)
assuming Lllin~Iin' With Vl-Vo=50mV, C=lOpF, Iin=lmA and LI lin = 100!lA (4.22) comes to tOl ~2.5 nsec.
If the input pulse length <>in is finite, two cases must be distinguished.
If <>in < tOl holds, the input current lin is cut off at the instant at which Ie<Iin is still valid (case (A) in Fig. 4.32d and 4.33b). Hence Ie-lin becomes negative, C will be discharged again, and the circuit returns to the initial state Vo. First if <>in>tOl ' the disconnection of lin results only in a diminution of Ie, but not in its polarity reversal, and the tran-sition process is continued (cases (B) and (C) in Fig. 4.32d) and 4.33b)).
Finally, if <>in is longer than the whole transition time (to 3), a small overshoot results (case (D)).
0in = .. step"
V1
a ~L-__ ~ ______________________ ~
o
~I;n = "high"
Fig. 4.33 a and b. Tunnel diode multivibrator pulse shapes
In order to trigger a tunnel diode discriminator therefore, two con-ditions must be fulfilled:
1. Alin>O must hold; the threshold given by lp must clearly be exceeded.
2. bin> t01 must hold.
Since t01 itself depends on lin, the second condition means that actually the whole integrated charge Qin of the input current pulse must be higher than about (V1 - vo)' C. Hence the tunnel diode dis-criminator is charge-sensitive for short pulses, and current-sensitive for long pulses (cf. [4.134]).
The shape of the output voltage pulse V D(t) is obtained by inversion of the relation (4.21). Unfortunately this inversion gives a non-linear differential equation which cannot be solved explicitly. Various graphical, numerical or approximative solutions are dis-cussed in the papers [4.041] to [4.043] and [4.135] to [4.137J.
In practical tunnel diode discriminator circuits, of course, it must also be taken into account that the output signal, too, runs into the in-put circuitry, since the inin-put and the outin-put are not separated. If for instance a TD discriminator is coupled by means of a coaxial cable to the photomultiplier, the cable must be terminated correctly at the photomultiplier output in order to avoid reflexions [4.044]' Alternatively the input of the discriminator can be separated from the detector out-put by means of a common base stage, as in Fig. 4.34 [4.045]' Of course, the collector of Q1 can be coupled directly into TD, without the capaci-tor Ck •
-
I141
~R4
9 *! K2l tCklt---<~-t---<>
VOU!~ 0*
I~ ~
- R
-.a:-L
Fig. 4.34. Decoupling of the TD trigger input by means of a common-base transistor stage
The dependence of the discriminator threshold on the pulse shape and duration can be reduced considerably if a window amplifier is con-nected in front of the tunnel diode. The circuit of Fig. 4.34 can be easily modified by the introduction of a diode D (dotted line connections) to
act as a window amplifier. Due to R4 , the diode D is forward-biased leading a current
h.
Only the difference lin - 14 is applied to the tunnel diode through Q l' RIGHINI [4.046J described tunnel diode discrimina-tors with biased emitter-followers as window amplifiers. WARD andYORK [4.047J used simple backward-biased semiconductor diodes for this purpose. In their circuit the discriminator threshold can be varied between 0.5 V and 10 V. Variation of the input pulse rise time between 0.5 and 20 nsec and of the pulse length between 5 and 100 nsec shifts the threshold by about only 10 m V. COLI [4.048J uses an additional input pulse shaping with the aid of a shortened cable at the discriminator input.
A voltage signal, of course, must be converted into a current signal, e. g. by means of a resistor (cf. R\ in Fig. 4.34), prior to triggering a tunnel diode discriminator. HVAM and SMEDSDAL [4.050] investigated a voltage-sensitive tunnel diode discriminator with TD in the high impedance collector circuit of a normal emitter-coupled and biased (---POT!) difference amplifier (Fig. 4.35). The output signal across the tunnel diode is picked up using a second difference amplifier. PANDARESE and VILLA [4.051] used tunnel diodes in bridge configuration for discriminators. Some other TD discriminator circuits are described in [4.138] and [4.139].
-1---~+
POT
Fig. 4.35. Principle of the TD voltage discriminator circuit given by [4.050]
With fast TD threshold discriminators, the fast single-channel ana-lyzer can also be realized, mostly by using one of the alternatives shown in Fig. 4.18. For the details ofthe related circuitry we refer to the original literature [4.018, 4.020, 4.140J and [4.022]' Because of the dependence of the output pulse length of a TD discriminator on the input pulse properties, an additional pulse shaping - commonly using another TD monos table multi vibrator - is needed prior to the anticoincidence circuitry.
Due to the dependence of the delay t01 between the input and out-put pulses on the inout-put pulse height, the time information is not well conserved in a tunnel diode discriminator. ORMAN [4.052, 4.053] de-scribed a TD discriminator for bipolar pulses exhibiting a highly ac-curate zero-crossover timing. Fig. 4.36 shows the simplified circuit of such a discriminator reported by ALSTON and DRAPER [4.054]. The operating point (A) lies above the peak current Ip. The discriminator fires (Point (1)) when the input current Iin(t) passes the threshold I v - 10 •
Because of 10';:::', Ip ' the return in the initial state (Point (2)) occurs ap-proximately at the zero crossing point of the input current. Since Ip is greatly exceeded, the transition time is very short. Hence the steep trailing edge of the output pulse can be used for timing purposes.
Any-+15V
1.5k
560
=
~P----"'-+ 10 V
ll"
560O~1
2N955-A=12
go
-10V---Fig. 4.36. Orman-type TD zero crossing discriminator according to ALSTON and DRAPER [4.054]
way, the discriminator has a fixed threshold Iv - I 0 ~ Iv - Ip' thus mak-ing a preconnected variable gain amplifier necessary. Other zero cross-ing discriminators with tunnel diodes are described by WIEGAND [4.055]
and GARVEY [4.056]'