Tiny Transistors used in wide-input Driver

A typical driver from a micro-controller output uses a transistor such as a 2N3904 or 2N2222. The logic level is inverted wrt the collector and ground, but is non-inverted for loads between the collector and the positive rail (high-side). This is a suitable way to increase the drive capability to 10s or 100s of mA and also buffer the micro-controller's outputs.

For higher currents in the 1-20A range, a mosfet is usually a good choice, with high currents and voltages and low losses. However the input control voltage is higher, often in the 5-10V range. There are some optimized for lower voltages, but there are few that have both ultra-low drive and high current. By adding an NPN transistor, the input control voltage is now just under 1V, suitable for 5V 3.3V or 1.8V mcus. The additional transistor adds an inversion which is is often undesirable, as the power-up state is ON. One more transistor provides the additional inversion. It requires a few more parts, but is a very robust and versatile driver circuit for lamps, relays, solenoids and other higher-current DC loads.

A dual transistor package can be a worthwhile trade to reduce the parts for a few more cents of cost.

Here's a circuit using the plain dual transistor as a driver. For lower currents (< 100mA), the load is connected to from Q1 to the Rail voltage (24V here; could be as low as 5V).

Dual Transistor non-inverting drive from logic input.

Vin and Vout; Response is fast

Adding a high current stage provides non-inverting high-gain drive signal from < 1.0V input all the way to the rail voltage (24V in this example). The mosfet output stage is suitable for driving multi-amp loads including inductive loads like relays As a guide, select a mosfet with at least 2x the rail voltage and  more than 2x the maximum continuous current.

 Logic drive from 1.2, 1.8, 3.3, 5.0V or higher, up to the positive rail voltage.

I have designed and built a few driver boards similar to the above circuit using the dual transistor below.

With a dual NPN: plain (no bias resistors): MBT3904DW1T1G or BC846BDW1T1G
 a dual based on the 2N3904 (TO-92) and MMBT3904 (SOT23-6)
  http://www.mouser.com/Search/Refine.aspx?Keyword=MBT3904DW1T

(See the MUN5333DW1T1G for the pre-biased version with even fewer components.

Update: identify tiny packages

ultra-small smd discrete transistor packages: SOT883, DFN0806-3

PMBT3904M,    NPN from NXP pkg: SOT-883

MMBT3906FA-7B,  PNP from Diodes Inc: DFN0806-3, X1-DFN1006-3

PBSS4112PAN, Dual NPN: DFN2020-6 (SOT-1118)