6bq5_EL84_spicemod.pdf

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PSpice Model
INTRODUCTION
6BQ5/EL84
This document supplements the PSpice model 6BQ5.INC, and provides some
background operation to the operation of the model along with details of functionality
modelled or not modelled as the case may be.
Whilst every care has been taken to duplicate the functionality of the modelled device
as described here, it should be stressed that modelling is not a substitution for
breadboarding or other prototyping methods.
No warranty of any kind is provided for this model, and no liability is assumed for any
damage or loss arising out of the use of this model, or application of the results of this model.
All trademarks acknowledged. The model is copyright ©1997-2003 Duncan Amplification,
and is made available for educational or non-profit use.
MODELLED FUNCTIONS
Inter-electrode capacitance, screen current, grid current to a limited degree. The grid
current is an approximation, and does not take into account grid current rise at low
values of Va and Vg2.
FUNCTIONS NOT MODELLED
A heater model is not implemented at this stage. Grid 3 is not modelled - it is
assumed to be connected to the same potential as the cathode.
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PSpice Model
MODEL PERFORMANCE
6BQ5/EL84
6BQ5/EL84 Pspice model - Transfer Characteristics
250
Vg2 = 300V
Vg1 = -14V to 0V
200
-2
0
150
Ia (mA)
-4
-6
100
-8
-10
50
-12
-14
0
0
100
200
Va
300
400
500
Figure 1: Anode current
Figure 1 above shows anode current against swept anode voltage for a range of grid
voltages between -14V and 0V in steps of 2V. Screen voltage in this instance is
300V.
The next diagram shows screen current overlaid on the same chart.
6BQ5/EL84 Pspice model - Transfer Characteristics
250
Vg2 = 300V
Vg1 = -14V to 0V
200
-2
0
Ia and Ig2 (mA)
150
-4
-6
100
-8
-10
50
-12
-14
0
0
100
200
Va
300
400
500
Figure 2: Anode and screen current together
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PSpice Model
6BQ5/EL84
As can be clearly seen, the screen current rises sharply Va is reduced.
MODEL DESCRIPTION
The following describes the various components of the model and their interaction:
Eat
is the arctangent calculation which causes the fall off in emission at lower
anode voltages.
is the emission contribution from the grid and screen, g
1
and g
2
.
is Egs after raising to the power of 3/2 and factored by a constant so that
it may be turned directly into a current value.
is the cathode current value. This is the current between anode and
cathode, although some of this may be diverted by the screen grid.
Basically consists of Egs2 multiplied by Eat.
is the actual cathode current. Synonymous with Ecath.
is the screen current value.
is the actual screen current. Synonymous with Escrn.
is the grid current value. This is an approximation at present, and will be
improved as more information becomes available.
Egs
Egs2
Ecath
Ga
Escrn
Gs
Gg
ALTERING THE MODEL FOR OTHER SIMULATORS
It may be necessary to use the model with other simulators, such as Berkeley SPICE
3f4, in which case some of the PSpice specific items will need to be altered.
The PSpice LIMIT{a,b,c} statement can, in instances where b is zero, be replaced by
the SPICE 3f4 statement URAMP(a). Where LIMIT{a,b,c} is used, with b=0 and
c=variable, the SPICE 3f4 statement U(a/c)*c can be used.
Duncan Munro
12th May 1997
Web page:
Forum:
Email:
http://www.duncanamps.com/
http://forum.duncanamps.com/
postmaster@duncanamps.com
6BQ5.INC - 12/05/97 r2
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PSpice Model
MODEL LISTING
6BQ5/EL84
*
* PSpice Subcircuit for 6BQ5/EL84 output pentode
*
* Supported:
screen current and interelectrode capacitances, also grid
*
current to a limited degree.
*
* Unsupported: Heater model.
*
* Note that the grid current is guesswork on my part in the absence
* of any data...
*
* D.Munro - 12/05/97
*
* 12/05/97 Initial model.
*
*
* Pins
A Anode
*
S Screen
*
G Grid
*
K Cathode
*
.SUBCKT 6BQ5 A S G K
*
* Calculate contribution to cathode current
*
Eat
at
0
VALUE={0.636*ATAN(V(A,K)/15)}
Egs
gs
0
VALUE={LIMIT{V(S,K)/19+V(G,K)+V(A,K)/400,0,1E6}}
Egs2
gs2
0
VALUE={PWRS(V(gs),1.5)}
Ecath cc
0
VALUE={V(gs2)*V(at)}
*
* Calculate anode current
*
Ga
A
K
VALUE={3.2E-3*V(cc)}
*
* Calculate screen current
*
Escrn sc
0
VALUE={V(gs2)*(1.1-V(at))}
Gs
S
K
VALUE={2.0E-3*V(sc)}
*
* Grid current (approximation - does not model low va/vs)
*
Gg
G
K
VALUE={PWR(LIMIT{V(G,K)+1,0,1E6},1.5)*50E-6}
*
* Capacitances
*
Cg1
G
K
10.8p
Cak
A
K
6.5p
Cg1a
G
A
0.5p
.ENDS
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