DC Holding Circuit Application Note TN#11

By Mike Stoos and Dave LeVasseur,
Wurth Electronics Midcom Inc. July 12, 1993
Revised 9/12/95

A DC holding circuit is used to "hold" a phone line in the active state by passing direct current while at the same time presenting a high impedance to ac signals. Our design goal was to meet the U.K. BS6305:1982 figure 4, (see also: NET4) 'voltage versus current' requirements and provide an impedance high enough such that effects on return loss would be negligible.

The basic circuit is a diode bridge connected to a biased darlington transistor in the common collector cascade configuration with an ac ground at the base. A zener diode from collector to ground provides over-voltage protection. It is recommended that attention be given to finding a zener with a sharp knee. A sharp zener knee will prevent deleterious effects on distortion and impedance.

The circuit can be stabilized by adding a capacitor C2 as shown. This lowers the ac impedance at higher frequencies, but raises it at the low end. For the four circuits tested, values between 6.1 nF and 7.3 nF gave the best results at 1000 Hz; therefore a standard value of 6.8 nF was chosen. Due to the capacitively complex impedance reference network required by many European countries, this capacitor may be increased in value and placed directly across the diode bridge, across the coupling transformer's primary or across its secondary to improve return loss.

 

Alternate Designs:	Using MPSA20/2N2102	Using MPSA10 darlington pair
Rf: 10 ohms	R1: 40.2k ohms	R1: 27.4k ohms
D1-D4: 1N4003	Q1: MPSA20	Q1/Q2: MPSA10 darlington
C1: 3.3uF, 35V	Q2: 2N2102	Re: 10 or 33 ohms
C2: 0.0068uF		Dz: must be a 17V or greater
Dz: 1N5243B		zener when Re=10 ohms
	Figure 1

 

Two 10 ohm resistors were added at the tip and ring terminals. They provide over-voltage protection and have the added advantage of increasing effective loop resistance. The increased resistance allows ample margin for the BS6305:1982 requirements. (A single 20 ohm resistor may be used instead)

While all four circuits met the specifications, the circuit using the MPSA20 and 2N2102 gave better results and seemed to be more stable then the circuit using the MPSA10. The circuit was found to have total harmonic distortion better than -70 dB and to be stable through a wide variation of the transistors' betas (hfe). The DC impulse response of the circuit also provides a measure of protection against bell-tapping.

While this application note was written to provide the reader with a general-purpose DC holding circuit, modifications may be necessary to provide optimum performance for a given telephone line interface. The user is encouraged to experiment with this circuit to better learn about its capabilities.

While analysis of the holding circuit is possible with the help of traditional circuit analysis programs, first-order transistor approximations made an exact solution impossible. It is possible to understand operation of the circuit in a general way by inspecting the circuit and noting that the darlington connected transistors can be treated as a single transistor having a very high beta and a Vbe twice that of a single transistor. Assuming for the moment that base current is negligible with respect to the current flowing in R1 and R2, the voltage at the base of Q1, Vb, is the ratio of the voltage divider created by R1 and R2. The voltage across Re becomes Vb minus the Vbe of Q1 and Q2 combined. (See figure 2)

Figure 2

 

For the MPSA20 circuit using Re = 33 ohms and assuming (just to start somewhere) Vc=4V, the voltage across Re becomes: VRe = 4(82.5/(40.2+82.5)) -2(0.65) = 1.39 volts Thus the emitter current is VRe / Re = 1.39V/33ohms = 42.1 mA. The collector current is essentially the same as the emitter current since the effective beta of the transistors is very high (10,000 to 20,000). Working backward through the diode bridge and the 10 ohm resistors yields an input voltage, Vi = 2(0.65V) + 42.1mA(10 ohms+10 ohms) + 4V = 6.14V. Actual measurements of 42 mA at Vi=6.53V were found which isn't too far off since the effects of base current were neglected to make our analysis simpler.

The ac analysis is easy to comprehend considering that the voiceband frequency impedance of C1 is very low with respect to the resistance (R2) that bridges it. At 1kHz, the impedance of C1 is about 48 ohms. Ignoring the phase angles and the effects of R2, the series combination of R1 and C1 form an impedance divider that make the ac voltage at Vb approximately equal to Vc(Xc1/R1), or Vc(.0012). From this, it can be seen that effects of the ac signal riding on the DC voltage at Vc will be greatly reduced at Vb. Since the ac current into Q1's base is a very weak function of the ac signal at Vc, ac variations at Ie, and hence Ic, will be minimal. The impedance of the circuit is the result of the miniscule change in Ic as a function of change in Vc. This ratio is the impedance, Z=dV/dI which tends toward infinity if Ic is invariant.

Here are the Voltage/Current and Impedance/Frequency measurements in tabular form:

Using MSPA20	Re=10 ohms	Re=33 ohms	BS6305:82, Fig. 4
I (mADC)	Vdc	Vdc	Vdc
0.00	2.21	2.20	0.00
25.00	4.37	5.21	9.00
33.5	4.73	5.86	10.00
42.00	5.11	6.53	12.50
45.00	5.25	6.74	32.00
98.22	7.40	10.67	10.71
105.66	7.73		7.74
125.00			0.00

Impedances seen at tip and ring using MPSA20, 2N2102, Re=33 ohms and Idc=50mADC:

Frequency (Hz)	Rs (kohm)	Xs (kohm)	Z (kohm)
200	1.543	5.261	5.482
250	2.312	6.433	6.836
300	3.234	7.539	8.204
400	5.606	9.445	10.984
500	8.665	10.791	13.839
600	12.374	11.228	16.689
750	18.291	9.372	20.552
1000	23.792	.001	23.792
1200	21.539	-7.576	22.832
1500	14.803	-12.704	19.507
2000	7.655	-11.603	13.901
2500	4.495	-9.746	10.733
3000	2.943	-8.213	8.725
3300	2.314	-7.404	7.757
3500	2.033	-6.993	7.282
4000	1.543	-6.163	6.353

 

IMPORTANT NOTE

Subsequent to the time this application note was written, the MPSA10 was discontinued by some manufacturers and the MPSA14 was found to be a suitable replacement.  The following parts list was based on use of the MSPA14 which provided results on par with the two solutions presented above.

DC HOLDING CIRCUIT PARTS LIST

(SURFACE MOUNT)

Newark

Phone: 1-800-463-9275

Fax: 1-402-592-0508

Digi-Key

Phone: 1-800-344-4539

Fax: 1-218-681-3380

Surface Mount MPSA14
Part	Stock Number	DistributorPrice (1k)	Distributor Price (100k)	Distributor
Q1,Q2: Darlington Pair MPSA14	MMBTA14DICT-ND	$0.186	$0.073	Digi-Key
D1-D4: DF02S Bridge Rectifier	DF02S-ND	$0.33	$0.275	Digi-Key
Dz: 1N5243B	ZMM5243BCT-ND	$0.1365	$0.049	Digi-Key
R1: 27k 5% 1/8W	CRCW1206273	$0.014	$0.006	Newark
R2: 82k 5% 1/8W	CRCW1206823JRT1	$0.014	$0.006	Newark
Re: 33 5% 1/2W	CRCW2010330JRT1	$0.06	$0.05	Newark
Rf: 20 5% 1/8W	CRCW1206200JRT1	$0.014	$0.006	Newark
C1: 3.3uF 50V electrolytic	PCE2030CT-ND	$0.188	$0.091	Digi-Key
C2: 6.8nF 100V ceramic	93F2405	$0.11	$0.04	Newark
C3: 3.3uF 50V electrolytic	PCE2030CT-ND	$0.188	$0.091	Digi-Key
		$1.2405	$0.687

 

DC HOLDING CIRCUIT PARTS LIST

(THROUGH-HOLE)

Digi-Key

Phone: 1-800-344-4539

Fax: 1-218-681-3380

Through Hole MPSA14
Part	Stock Number	Dist. Price (1k)	Dist. Price (100k)	Distributor
Q1,Q2: Darlington Pair MPSA14	MPSA14-ND	$0.1162	$0.078	Digi-Key
D1-D4: 1N4003	1N4003CT-ND	$0.036*4=0.144	$0.015*4=0.06	Digi-Key
Dz: 1N5243B	1N5243BCT-ND	$0.061	$0.028	Digi-Key
R1: 27k 5% 1/4W	27KQBK-ND	$0.004	$0.004	Digi-Key
R2: 82k 5% 1/4W	82KQBK-ND	$0.004	$0.004	Digi-Key
Re: 33 5% 1/2W	33HND	$0.007	$0.007	Digi-Key
Rf: 20 5% 1/4W	20QBK-ND	$0.004	$0.004	Digi-Key
C1: 3.3uF 50V electrolytic	P6262-ND	$0.035	$0.028	Digi-Key
C2: 6.8nF 100V ceramic	1393PH-ND	$0.031	$0.028	Digi-Key
C3: 3.3uF 100V electrolytic	P6291-ND	$0.042	$0.035	Digi-Key
		$0.4482	$0.276
Add approximately $0.01 for		$0.01*13=0.13	$0.01*13=0.13
installation of each part. Cost	including assembly	$0.5782	$0.406

Reference only

Distribution pricing, not OEM