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ULTRA HIGH-SPEED ICs
By Klaus Lehmann, Burr-Brown International GmbH
1
OPA660 Current
Feedback Amplifier
OPA622 Voltage
Feedback Amplifier
Out
Out
2
OPA660 Straight
Foward Amplifier
+In
Out
3
+In
+In
–In
4
5
QUASI-IDEAL CURRENT SOURCE
In addition to their actual operation parameter
transconductance, active electronic key components such as
vacuum tubes, field effect transistors, and bipolar transistors
demonstrate diverse negative parameters. In applying the socalled Diamond structure, the user can obtain an improved
current source with reduced disturbance parameters, as well
as a programmable transconductance independent of temperature. Standard applications for the Diamond current
source (DCS) can be found in buffers, operational amplifiers
with voltage or current feedback, and transconductance
amplifiers. The DCS simplifies the design of electronic
circuits with bandwidths of up to 400MHz and slew rates of
3000V/µs with a low supply current of several mA.
6
COMPONENT
PARAMETER
TYPICAL VALUE
Triode
Grid Bias Voltage
Anode Bias Voltage
Grid Current
Anode Bias Current
G/K Resistance
A/K Resistance
Trans Grid Action
0 to 10V
20 to 1kV
nA to µA
µA to A
kΩ to MΩ
kΩ to MΩ
1 to 20%
N-J FET
Gate Voltage
D/S Voltage
Gate Current
D Bias Current
G/S Resistance
D/S Resistance
Inverse Amplification
0 to –10V
0 to 100V
fA to µA
µA to A
MΩ to GΩ
kΩ to MΩ
1 to10%
NPN Transistor
Basis Voltage
K/E Voltage
Basis Current
K Bias Current
B/E Resistance
K/E Resistance
Inverse Amplification
0.5 to 0.8V
0.5 to 100V
µA to mA
µA to A
kΩ
kΩ
0.1 to 1%
VOFF1
VOFF3
IBIAS1
IBIAS3
R12
R32
VR31
–2 to +2mV
0V
nA to µA
µA
kΩ to MΩ
kΩ
<0.1%
VOLTAGE-CONTROLLED CURRENT SOURCES
For analog signal processing, especially current or voltage
gain, previous electronic circuit techniques primarily used
vacuum tubes, while today they use field effect or bipolar
transistors. The triode illustrated in Figure 1 is representative of the various vacuum tubes, while the N-channel FET
represents the FET variations (junctions, insulated gates,
depletion, enhancements, P-channels, and N-channels), and
a NPN transistor represents the range of bipolar transistors.
Triodes, N-J FETs, and NPN transistors are compared with
the Diamond current source (DCS). The common elements
of all of these active elements are a relatively high-impedance input electrode 1 (grid, gate, basis), a low-impedance
©
1993 Burr-Brown Corporation
SBOA070
AB-180
DCS
TABLE I. Typical Disturbance Parameters of the VoltageControlled Current Sources.
Printed in U.S.A. May, 1993
describes the change of the output signal (VOUT) dependent
upon the input signal (VIN).
input and output electrode 2 (cathode, source, emitter), and
a high-impedance output electrode 3 (anode, drain collector). Thus all of these elements can be treated as special
voltage-controlled current sources (VCCS = Voltage-Controlled Current Source). The limitation “special” refers to
the low-impedance input and output electrode 2. The most
important relation between the electrodes 1, 2, and 3 is the
transconductance gm. For instance, the transconductance
Triade
3
1
G
–gm
K
Ia = K (Vgk +
gm =
dIa
dVgk
+
G
2
Id = β (1 + λVds) (Vgs – Vp )2
gm =
dId
dVgs
gm =
= 2β (1 + λVgs) (Vgs – Vp )
dTc
dVbe
Vbc
VAF
=
–
Is
VT
–gm
2
2
Vbe
VAR
) (eVbe /VT – eVbe /VT)
I3 = V12 x gm
gm = 2K
eVbe /VT
Ig
VT
K = 0.81; VT = 25.9mV
Is = 1.58E –16; VAF = 66; VAR = 3
β = 2.258E –3; λ = 21.31E–3; Vp = –2
K = 0.001; D = 0.05
Ia
(mA)
Vgk
30
25
20
15
10
5
–1V
–2V
–3V
–4V
–5V
Vak
Ia
Vak
40V 60V
80V 20V
100V
–Vgk
gm
6
4
60V
80V
1
2
3
(V) –5 –4 –3 –2 –1
5
Vds
+5V
+2.5V
–Vgs
–1.5
–1
gm
Vds
+5V
+2.5V
3
1
(V)
(mA)
(mA)
10
10
8
8
6
6
4
4
2
2
(V) –2
(mA/V)
(mA/V)
–1
3
10
8
4
–I3
1V...5V
–V12
(mV)
Vbe
0.72
(mA)
Ig
2.4mA
1.2mA
0.6mA
0.3mA
–5
+V12
+20 +40 +60 (mV)
(mA)
(mA/V)
160
5V
3V
1V
120
40
Vbe
0.80
–60mV
(mA)
80
0.76
(V)
–20mV
–60 –40 –20
(gm)
100
+V32
+3
+5
–10
–I3
1 (V)
gm
Vce
+2
+10
Vce
200
–0.5
+1
–10
(V)
5
+I3
300
6
–1
–40mV
Ic
400
2
–1.5
2
–2
+20mV
–5
Vce
0.2 0.4 0.6 0.8
4
–Vgs
(V) –3
0.786V
V12
+40mV
+5
–V32
0.804V
2
–0.5
(mA/V)
1
20V
4
Id
2
100V
4
Vds
+60mV
Vbe
0.828V
0.822V
0.815V
6
–1.1V
(mA)
+10
8
–Ø.73V
2
4
40V
Vgs
±ØV
–Ø.21V
–Ø.45V
8
(V) –2
+I3
Ic
10
(mA)
12
10
8
6
4
2
(V) –5 –4 –3 –2 –1
Vak
(mA)
Id
10
100 150 200 (V)
50
Ic = Is (1 –
1
–gm
E
2
3
3
1
B
Id = βVds (1 + λVds) [2 (Vgs – Vβ ) – Vgs]
K Vgk + DVak
DCS
C
–gm
gm
3
DVak) /2
3
3
1
2
Vgk < Ø (Vgk + DVak) > Ø
–Vgk
NPN - Transistor
D
(1)
To operate each VCCS, it is necessary to adjust the DC
quiescent current or voltage individually (see Figure 1).
N - J - FET
A
(mA)
VOUT = VIN x gm x ROUT
0.84 (V)
–V12
Ig
2.4mA
1.2mA
0.6mA
0.3mA
+V12
(mV) 60 40 20 – + 20 40 60 (mV)
FIGURE 1. Comparison Between Voltage-Controlled Current Source (VCCS) and Diamond Current Source (DCS).
2
Input
Signal
Real VCCS
Ideal VCCS
Input
Compensation
Output
Compensation
Output
Signal
VOUT
3
VOFF1
– +
1
V'OFF1
V'r31
+ –
+ –
1'
R'12
IBIAS1
+ –
V'OFF3
VOFF3
gm
2'
VIN
– +
3'
I'BIAS1
R'32
RL
I'BIAS3
R'S
1/
gm
IBIAS3
2
FIGURE 2. Internal and External Substitute Circuitry of a Voltage-Controlled Current Source.
Figure 2 illustrates the inner and outer substitute circuitry of
a voltage-controlled current source VCCS. According to the
circuitry, the VCCS (1, 2, 3) consists of an inner ideal VCCS
(1', 2', 3') with transconductance gm and a row of inner
disturbance parameters (V', I' , R'), which determine, among
other things, the adjustment of the DC point. Table I shows
a rough overview of the disturbance parameters. Almost all
disturbance parameters are subject to tolerances between
units and show dependent temperature behavior.
mond circuit, illustrated in Figure 4, opens up the possibility
of implementing the quasi-ideal VCCS [2]. In the ideal case,
in which NPN and PNP transistors are identical, the disturbance parameters V0FF1', VOFF3', IBIAS1', and IBIAS3' disappear.
But in real circuits, of course, this is not the case. The
remaining parameter values are, however, much smaller in
comparison with a conventional VCCS (compare VCCS
with DCS in Figure 1 and Table I). In the modulation range
being examined, from I3 = ±10mA, the transconductance
varies from 120 to 160mA/V as opposed to 0 to 350mA/V.
This means that the improved VCCS (designated DCS from
now on) causes a reduction in signal distortion.
Figure 2 also illustrates the correction parameters (VOFF1,
VOFF3, IBIAS1, and IBIAS3), which are required primarily to
compensate the internal disturbance parameters. The correction parameters, however, do not correct the effects of the
internal disturbance parameters (R'12, R'32) and the output
voltage feed-through V'r31. Roughly stated, at least 50% of
the design time for electronic circuit techniques goes toward
dealing with the problem of compensation. Thus in complex
circuits, the connection between the function parameter gm
and the various disturbance parameters requires more and
more modifications in circuit variations. If a VCCS without
disturbance parameters was available for users, the huge
variety of electronic circuit techniques could be reduced.
1
VIN
2
OTA
3
B
VOUT
R2
Rf
FIGURE 3. Operational Amplifiers as Series Connection
Between OTA and Buffer.
THE “IDEAL” CURRENT SOURCE
The macro element operational transconductance amplifier
(OTA) and operational amplifier (OA) contain circuit parts
for reducing the previously mentioned disturbance parameters. The feedback operation necessary with these amplifiers— i.e. the application of a control loop with its unavoidable delay time (phase delays)— causes significantly reduced time and frequency domain performance compared to
the VCCS. Straight-forward amplifiers are thus more widebanded than feedback amplifiers. An operational amplifier
OA, as shown in Figure 3, consists of the series connection
of an OTA with a buffer B. The OTA is a voltage-controlled
current source VCCS, in which the electrode 2 can be used
“only” as a high-impedance input. Because of this distinction, the OTA can only be used with an external feedback
loop. In contrast to conventional operational amplifiers with
voltage feedback as shown in Figure 3, the current-feedback
OA contains an OTA with low-impedance input and output
2—i.e. the previously represented “ideal” VCCS. The Dia-
VCC
I'Q
3
3
IO
1
1/
6
1/
2
3
6
IO
3
3
I'Q
VEE
FIGURE 4. VCCS with Diamond Structure.
3
PROGRAMMABLE TRANSCONDUCTANCE
Conventional VCCSs allow the transconductance to be
adjusted depending upon the quiescent current. In the DCS,
the transconductance is adjusted primarily with the current
sources I'Q (see Figure 4). For this adjustment, one effective
method is to create a current source control (Figure 5).
are available for the DCS (Figure 7): Buffer (B), CurrentFeedback Transconductance Buffer (TB), Transconductance
Amplifier (TA), Direct-Feedback Transconductance
Amplifier (TD), Current-Feedback OA (TCC), and VoltageFeedback OA (TCV).
Using the resistor RQ, the quiescent current I'Q or IQ and thus
the transconductance gm can be fixed. The temperature
function of gm (due to VT = f(T)) is compensated for by
corresponding variations of IQ. For RQ → ∞, IQ → 0 and gm
→ 0, and VCCS is switched off. In contrast to the conventional VCCS, the DCS functions in two quadrants at the
input and in four at the output. In the VCCS, the
transconductance is fixed by the choice of DC points within
the usable modulation range, while the transconductance in
the DCS is largely independent of the modulation and can be
adjusted with the external resistor RQ.
OUTLOOK
To characterize typical dynamic coefficient values (Table
II), a developed DCS including a SOI package was simulated in the circuit shown in Figure 8. Burr-Brown brought
this DCS onto the market as OPA660.
LITERATURE
[1] Ross, D.G. et al; IEEE Journal of solid-state circuits
86, vol. 2, p. 331.
[2] Lehmann, K; Elektronik Industrie 89, vol. 5, p. 99.
Strom-oder Spannungs-Gegenkopplung?
(Current or Voltage Feedback? That’s the question
here.)
gm = dI3/dV12 is negative for all VCCSs. In contrast, the
transconductance gm = dI3/dV12 of the DCS is positive. As
previously mentioned, the following standard applications
VCC
3
1
1
1
VT
I'Ø =
RØ
I'O
I'O
gm =
I'O
I'O
2KIØ
VT
=
1
2K X 18I'Ø
RIN
1
+gm
2
ROUT2
+gm
2
VOUT
VT
2K X 18 X In (10)
gm =
10
VIN
In (1Ø)
1
RØ
FIGURE 8. Circuit for Recording the Dynamic Characteristic of a TCC with DCS.
1
RQ
VEE
FIGURE 5. Current Source Control with Adjustable Bias
Current.
+V
4
3
1
(mA/V)
2
6
RQ
100
–V
(gm)
10
RQ
1
0.1
1
0.1Vp-p
f–3dB
(MHz)
6Vp-p
f–3dB
(MHz)
4Vp-p
SR
(V/µs)
1.4Vp-o
DG
(%)
5MHz
DP
(Degrees)
2.4
1.2
0.6
0.3
400
240
140
80
330
200
100
55
2850
1750
800
420
–0.07
–0.06
–0.05
–0.03
–0.05
–0.06
–0.10
–0.19
TABLE II. Typical Dynamic Values of a TCC with DCS
Corresponding to the Circuit in Figure 8.
+gm
1000
5
IQ
(mA)
10
(Ω)
FIGURE 6. The Relations gm = f(RQ) and Block Diagram of
the DCS.
4
B
TB
3
VIN
1
1
VIN
+gm
2
RIN
VB = 1/[1 + 1/(gm x RIN)] = 1
3
Ri2 = 1/gm
+gm
VTB = 1/[1 + 1/(2gm x RIN)] = 1
2
RIN
V'OUT
TA
TA
V'OUT
VOUT
3
VIN
1
+VTA = ROUT/(RIN + 1/gm) = ROUT/RIN
3
1
ROUT
+gm
2
RIN
Ri2 = 1/2gm
VOUT
+Ri3 = ROUT
ROUT
+gm
–VTA = +VTA
2
RIN
–Ri3 = ROUT
VIN
TC
TC
VOUT
3
VIN
1
1
+gm
+gm
ROUT
RIN
ROUT
3
3
1
+gm
1
+gm
2
ROUT
ROUT
1
2
ROUT
2RIN
RIN (ROUT/2 – 1/(2gm)
RIN + 1/(2gm)
2
–VTCC = –
=
ROUT
2
ROUT
RIN
ROUT
RIN
VOUT
TCV
1
+gm
+gm
3
+gm
1
=–
+gm
3
VIN
2RIN
+VTCC = 1 +
VIN
VOUT
1
+gm
2
RIN
TCV
RIN + 1/(2gm)
Ri3 = ROUT –
TCC
2
RIN
ROUT
2
TCC
1
=1+
ROUT/2 – 1/(2gm)
–VTC =
VIN
VIN
ROUT/2 + RIN
RIN + 1/(2gm)
3
2
RIN
+VTC =
VOUT
ROG
1
2
ROUT
2
+gm
1
VOUT
2
RIN
ROG
1
3
+VTCV = 1 +
2
–VTCV = –
RIN
VIN
FIGURE 7. Standard Applications with the DCS.
5
ROUT
VOUT
ROUT
RIN
ROUT
RIN
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